LONDON -- When Mikhail Gorbachev stopped here on his way to the Washington summit last month, Prime Minister Margaret Thatcher gave him a Christmas present. A clock with three faces, it showed Washington time on one side, and Moscow time on the other. The third and middle face, by some accounts a bit larger than the other two, told the time in London.

In ways that go far beyond geography, the gift was a metaphor for Thatcher's view of Britain's place in the world. She frequently cautions that there are only two superpowers, modestly noting that Britain is not one of them. But increasingly these days, neither Gorbachev nor Ronald Reagan takes a step toward the other without first checking the time, and the temperature, with Maggie.

Even if she had nothing else to her credit, Thatcher is now the West's senior statesperson through sheer longevity. Having sat at the table with Jimmy Carter and Reagan, Helmuts Schmidt and Kohl, Giscard and Mitterrand and the rest, she stands a plausible chance of outlasting even the next round of alliance leaders.

Today Thatcher becomes Britain's longest-serving prime minister this century, passing the mark set when Herbert Henry Asquith's eight years and 241 days in office were ended by David Lloyd-George in 1916. By the time her current term runs out, in 1992, she will be only 67, younger, she points out, than some of her alliance counterparts when they first assumed office. Without yet committing herself to a fourth run, she has said she feels as if she could go "on and on." There is little visible on the political horizon here that seems capable of stopping her.

But there is more to the Thatcher phenomenon than staying power. Under her stewardship, Britain has shed its image of a strike and debt-ridden disaster area and gained a reputation worldwide for prudent and parsimonious management. She has virtually eliminated the twin evils she and many others blamed for much of the nation's earlier decline -- trade-union power and state-owned industries, legislating the former into submission and selling off or shutting down the latter.

Her European Community partners may grumble at Thatcher's imperious ways, but they've learned to respect both her power and her perserverance. It is Thatcher to whom Gorbachev turns when he wants a stimulating argument or commiseration over the problems of revamping a reluctant, aging economy. It is she on whom the other western leaders depend to keep Reagan within the bounds of alliance consensus.

Thatcher has become the leader that Reagan pretended to be. Far from the puny victory in Grenada or the disastrous losses of Lebanon, she fought and won a real war in the Falkland Islands. She held her ground against terrorism, refusing to trade even soft words for hostages. Never let it be said that Thatcher met controversy with a quavering "I don't remember." A curt "none of your business" is much more her style.

She has had a lot of lucky breaks along the way. A deeply split political opposition, seemingly bent on self-destruction, has allowed her to win large Parliamentary majorities with only about 42 percent of the vote in each of three straight elections. Windfall oil profits, along with the proceeds from state-industry sales, gave her room to cut both personal taxes and public borrowing without reducing government spending. The well-timed Falklands conflict with Argentina was a massive vote-winner, appealing to deep strains of jingoism here.

Yet a great many Britons -- many within her own Conservative Party -- don't like Thatcher. In poll after poll through the years, she has been seen as cold, driven and uncaring. The "Leaderene," "Attila the Hen," "Rhoda the Rhino," and "Nanny" are just a few of the nicknames by which she is known -- and they're the nicer ones. In vast swaths of the country, particularly in the depressed industrial north, the mere thought of her inspires seething hatred.

Even among those who have benefited from her policies -- the seven million new shareowners of privatized companies, the million families who have purchased public housing she put on the market -- there is a deep-seated uneasiness over the other millions she has put out of a job by squeezing the economy. Many have seen their personal income taxes cut by her government. But most, according to polls, would rather have the money spent on the ailing National Health Service.

Yet of all the "isms" of the 1980s -- Reaganism, monetarism, even conservatism -- it is Thatcherism that has survived most intact. Often strident, sometimes bullying, always confident, it is more attitude than ideology. It is best defined by the figure of the PM herself, striding firmly forward as she tries to remake Britain in her own image -- self-sufficient, brisk, frugal and hard-working, full of 20th-century energy and 19th-century values.

More than even her backers thought possible, she has yanked this country back from what she considers its unnatural dalliance with socialism and changed the terms of political debate. Even the Labor Party has stopped talking about buying back the companies and houses Thatcher's government has unloaded or taking away the trade unions' new secret ballots.

Handbag hanging firmly on her elbow, pearls swinging from her neck, Thatcher doesn't walk so much as she bustles, her head and upper torso frequently out in front as her feet -- and often her aides -- struggle to keep up. She barely sleeps and frequently works an 18-hour day. While never known as a great intellect, she has an encyclopedic memory for everything from the number of Soviet missiles in eastern Europe to the number of hospital beds in western Wales.

She seems genuinely disappointed when Britons revert to their more traditional compromising and inefficient ways. When the jobless reject her poorly-paid training schemes in favor of the dole, or physicians protest against her mean treatment of the health service, she often reacts, in the words of columnist Neal Ascherson, "like Walter Ulbricht of East Germany who -- in Brecht's taunt -- wanted to dismiss his population and appoint another."

Who is Margaret Thatcher, and where did she come from? Peter Riddell, the political editor of the Financial Times, recalls that when she was chosen in 1975 to lead the Conservative Party, the most that could be said of her was that she was not Edward Heath. Despite two straight electoral losses, Heath was determined to stand again for selection as party leader. But the party rank and file was groping for a new direction, and into the breach stepped Thatcher.

Thatcher, who had served as education secretary, was not a major figure. Her strength, and many of her ideas, came from Heath's health and social services secretary, and Britain's New Right guru, Keith Joseph. Their shared analysis was that Heath had failed because he had fallen, like his Tory predecessors, into the trap of capitulation to the unions, state intervention and trying to spend his way out of trouble. As the traditional party grandees stepped aside, Thatcher, a commoner, was swept into the leadership by what Conservative Member of Parliament Julian Critchley caustically called the "Peasant's Revolt."

Apart from the trained modulation of her once-shrill voice and Midlands accent, and a more sophisticated wardrobe, Margaret Thatcher has changed little since 1975 -- or, indeed, since her school days in the provincial market town of Grantham. Her mother was a dressmaker, her father a relatively prosperous shopkeeper above whose grocery the family lived. From her father she learned a stern moralism, a belief in the supreme value of individual enterprise and hard work, and a strong dislike of the union movement that was growing by leaps and bounds in prewar Britain.

She studied chemistry at Oxford University, while heading the Conservative student organization and holding off-campus jobs. After graduation, she worked at research jobs, while managing to take law courses and to twice contest -- unsuccessfully -- a Conservative parliamentary seat. In 1951, much to the surprise even of close friends, who wondered where she had found the time for courtship, she married Denis Thatcher, a divorced businessman 10 years her senior. In 1953, five months pregnant with twins, she passed her intermediate bar exam. Four months after their birth, she passed the final.

By 1959, when Thatcher finally entered Parliament, the Tories were riding high under Prime Minister Harold Macmillan. Yet as the 1960s began, the economy began to slip. The minority right wing watched in dismay as Macmillan struggled, unsuccessfully, to maintain full employment by intevening in industry and boosting public spending.

Both Thatcher and Keith Joseph, now retired from her cabinet, believe that Macmillan's failure to stick to conservative principles led directly to national decline, Labor governments and, ultimately, the rise of Thatcherism. But the disastrous road that Macmillan, and later Heath, followed, Joseph said in an interview, began long before, when turn-of-the-century Tory prime minister Lord Salisbury first capitulated to the rising power of the then-new trade unions. Salisbury "wept about the unions. He said, 'We've given them such immense freedom under the law . . . and they will destroy their own people and the country.'"

That, Joseph said, was the beginning of what he calls the ratchet effect. "Whatever Labor said, the Tories broadly accepted and built into their own program? Why? Because it was hopeless to argue the cause, the unfashionable cause of self-reliance and competitiveness. If I had to commit myself to a single sentence, what Thatcherism represents is the rejection of the ratchet argument . . . . She said, 'There ain't no more ratchet.'"

Like their counterparts on the Republican far right during the same period in the United States, the Thatcherites recognized that their party, and their country, were ripe for major political upheaval. Like the Reaganites, Thatcherites had a built-in bias against the traditional party elite. Like Reagan, Thatcher came to office with a sense of mission; where he promised that America would stand tall, she pledged to make Britain great again.

But Thatcher's approach to righting her national economy was manifestly different from Reagan's. Thatcher, like the penny-pinching housewife she professes to be, refused to spend what she did not have. No "supply-side miracles" for her -- she believed that tax cuts would spur the economy, but any revenue boost was to be used to cut borrowing. When her cabinet came to her in 1981, demanding more money, she answered by raising indirect taxes.

Her squeeze on the economy led to a deep recession and, by 1982, many supporters doubted she could be reelected. "Inflation was still pretty high, unemployment was rising," recalled Lord Whitelaw, her deputy prime minister. "The Falklands war came at a crucial moment in her fortunes. Would we have won the 1983 election without it? Nobody could tell." The 1983 campaign was also fought with the critical advantage of a weak and left-wing Labor opponent, just as her third electoral triumph last June was ensured when Labor and the third-party Alliance split the opposition vote between them.

Despite her unbending reputation, Thatcher's record show her policies have not been as consistent or successful as she would have Britain believe. Although her government still pays occasional lip service to the glories of monetarism, it has largely abandoned it as policy. For all her talk about squeezing the public purse, government spending in 1987 took about the same 40 percent of gross national product that it did in 1979. While personal taxes have gone down, the total tax burden for most Britons is about the same as when she took office.

The rate of inflation is down, but still high by European standards. Per capita production has soared, but total manufacturing output remains below its 1979 level. Striking miners were brought to heel last year, but high wage settlements are still a problem. With the value of the pound climbing, Britain's trade balance has gone from a healthy surplus in 1985 to a 1987 deficit that will top $4 billion.

Still, as former Tory party chairman Norman Tebbit points out, Thatcher doesn't necessarily need to be consistent, nor even successful, to stay on top. With her challengers in disarray, she simply needs to be perceived to be a strong leader. Tebbit admits that if she had had to call an election in early 1986 when cabinet upheavals hurt her popularity, she might well have lost.

Instead she waited until last June when she was again in tight control. Her cabinet and her domestic leadership image intact, she was fresh from her spring visit to Gorbachev in Moscow, and headed for the western summit meeting in Venice.

Thatcher's strongest interests still lie in the domestic arena -- her recent radical initatives to curb the powers of Britain's Labor-led local governments have alarmed her critics and even some of her followers. But as her foreign policy forays have enhanced her image at home, her hold on domestic power has enabled her to turn more attention outward. In 1984, she faced down virtually the entire Common Market in demanding a better financial deal for Britain and since then she has become the strongest -- if also, to many of her European colleagues, the most irritating -- voice for European Community fiscal prudence.

Thatcher is not a foreign policy innovator; her attempts to push the Americans and Israelis to a Middle East peace conference have met with little response. Her influence in world affairs is felt primarily on regional, alliance and East-West issues in defining the limits of debate and lending counsel and approval to the initiatives of others.

She has played an expanding role as intermediary between Moscow and Washington, always inserting her own views into the dialogue, and is proud of her close relationships with both Reagan and Gorbachev. As the NATO leader with the longest time in office -- and the one most likely to remain there for the foreseeable future -- she is the closest link between America and its European partners. Carried forward by the sheer force of her personality she has become impossible to ignore.

When Reagan aroused allied concern with his Strategic Defense Initiative, it was Thatcher who flew to Washington to rein him in. She repeated the trip after the Reykjavik summit, emerging with assurances that Reagan would not soon seek to rid the world of nuclear weapons.

Her agreement to the finally brokered U.S.-Soviet treaty eliminating intermediate-range nuclear forces (INF) from Europe was sought by both Gorbachev and by Reagan. In return for it, she led western European demands for early new negotiations to reduce conventional weapons and chemical weapons.

The administration considers her active approval crucial to Senate ratification of the INF accord. Presidential candidate Robert Dole telephoned Thatcher to make sure she backed the treaty before announcing his own position shift in its favor.

Her close personal relationship with Reagan has been "a crucial factor for both," said Whitelaw. But as Reagan's strength has waned, and hers has grown, he said, "it's turned around. She needed him very much earlier on. Now, I think, he needs her."

Karen DeYoung is The Washington Post's London bureau chief.