LONDON -- No Western leader has done more with less than Margaret Thatcher. In the past eight years she has engineered an economic and social revolution in Great Britain without ever gaining majority support for her leadership. Now she has won the right to expand and extend her program into the 1990s -- even though a large majority of voters in this democratic society again used their ballots two weeks ago to register a loud dissent.

The awe-struck tone of most of the commentary on both sides of the Atlantic about Thatcher's victory ignores the extent to which she has benefited from the antic British election system. The ''first-past-the-post'' system of giving each seat in Parliament to the top contender, with no majority requirement or runoff provision, can form a huge parliamentary majority out of a popular-vote plurality in a multi-party contest.

Thatcher's Conservative Party candidates have never gained as much as 44 percent of the popular vote in any of the last three elections. With opposition divided among Labor and the Alliance of Liberals and Social Democrats, popular-vote pluralities of about 43 percent have yielded her parliamentary majorities of 44, 144 and now 102 seats -- enough to rubber-stamp most of her program.

She has defied the maxim that bold policy cannot rest on a narrow electoral base and has shown that, under certain circumstances, bold leadership may be enough in itself. Curbing union power and shifting major businesses from state control to private ownership have proved to be popular policies. But she has taken many other actions in housing, health and welfare in the face of strong public opposition, and allowed unemployment to rise to record levels rather than risk deficit spending or the revival of inflation.

It is quite a record, and she shows no sign of slowing the pace. When Parliament reconvenes this week, she is expected to propose a new version of tax reform, cutting basic income-tax rates and substituting a highly controversial per-capita poll tax for the local property tax, thus radically shifting the burden of local services off the backs of property owners. She will also propose other legislation weakening the grip of big-city local authorities (still largely controlled by the Socialists) on schools, housing and economic development in their jurisdictions. Her allies call it carrying the fight to the enemy.

No one of any prominence here even suggests that Thatcher ought to moderate her course or reassess her priorities in deference to the 57 percent who voted against her party. Whatever she does will be accepted as legitimate. But acceptance is not acquiescence. And the social and political divisions in Thatcher's Britain are evident to everyone except those American journalists who see only the growing affluence of the parts of London they visit.

The electoral map shows Tory strength rising across the Southeast (where those with good jobs speak with smug satisfaction of regional unemployment being cut to 7.6 percent) and lapping into some of London's rapidly gentrifying neighborhoods. But the north of England, Scotland, Wales and most of working-class London and the old industrial cities, all with double-digit unemployment, have sent a resounding no to Thatcherism. Millions of Britons in these areas are effectively without influence on government for another five years.

Their frustration focuses understandably on the opposition parties whose weaknesses play into Thatcher's hands. The Labor Party -- whose refusal to confront trade-union excesses in the late '70s set the stage for Thatcher's rise -- is showing signs of life. Its young leader, Neil Kinnock, had the best campaign of any of the party leaders, acquiring the stature of a potential prime minister.

But Kinnock is hobbled by a far-left element which gained parliamentary strength and by his personal adherence to a vision of unilateral disarmament which his own pollsters have told him cost Labor a vital 5 percent swing vote. Kinnock probably has the power to win a battle with ''the loony Left'' before parliamentary candidates are chosen for the next election. Whether he has sufficient ambition to adjust his conception of Britain's NATO role to the sensibilities of the British public is something on which his political intimates divide.

Clearly Labor has turned back the challenge of the Liberal-SDP Alliance as the principal vehicle for opposition to Thatcher. Labor won only 9 percent more of the popular vote than the Alliance but came away with 229 seats to the Alliance's 22.

The SDP wing of the Alliance -- a breakaway group of Laborites viewed by many Americans as a splendid set of pro-NATO economic and social moderates -- saw three of its four founding members go down to defeat. The only survivor, David Owen, is accused even by longtime allies of acting as a spoiled child, throwing up objections to a formal merger with the Liberals. But even a merged third-force Alliance looks a weak bet to displace Labor.

For now, and the foreseeable future, Britain is in the hands of Margaret Thatcher, enjoying -- or enduring -- the ministrations of the most powerful minority prime minister and the boldest narrow-base government in the West.