"The reason I am in politics," Margaret Thatcher said as she leaned forward from the edge of her chair to stare intently into her interviewer's eyes, "is because I believe in certain things and try to put them into practice."

What she believes in and has become an evangelist for are Victorian English virtues: industry, charity, selfreliance and duty. She considers them cardinal principles for which she is determined to fight at any political cost. Critics and even close colleagues speak instead of stubborn prejudices and self righteous combativeness.

But after more than three years as Britain's prime minister, Thatcher said she has no doubts or regrets. Her resolve has been tested first by the long unpopularity of her survival-of-the-fittest economic policies and now by prosecution of a completely unexpected Victorian colonial war in a faraway place for national honor, pride and principle.

Asked during a 70-minute conversation yesterday in a second-floor study of 10 Downing St. on a humid, early summer day what had surprised or troubled her during an unusually eventful and controversial period in office, Thatcher fell uncharacteristically silent. After thinking for a moment, her chin in her hand, she said, "I've fallen so well into it now, one's learned to cope. I can't think of anything that's surprised me."

Asked about her accomplishments, Thatcher listed a reduction in inflation, abolition of various government economic controls and increased efficiency of some British businesses. But she said three years were not nearly enough time for the kind of fundamental changes she wants to make in Britain. "Maybe 23 years," she said, with only a trace of a smile, as she talked of the necessity of a minimum of two five-year terms.

This is the selfassured foe facing both the Argentine military junta in the Falkland Islands and currently beleaguered opposition politicians here in Britain. Described by one close associate as someone who thrives on crises while having little longrange vision, Thatcher appears to have been as personally energized by the Falklands war as she and her Conservative Party have been buoyed by it in public opinion polls.

A new poll by the respected Market Opinion Research International firm published today showed that public satisfaction with Thatcher's government has jumped from 18 percent last December to 49 percent this week. Thatcher's own overall performance as a prime minister is now approved by 58 percent of the voters, compared with about 80 percent approval of her handling of the Falklands crisis.

Thatcher said she realizes from a 30year career in politics not to read too much into the recent surge of patriotic support for the government in the polls. Anyway, she said, "I wouldn't dream" of calling a quick election to try to capitalize on this support after the Falklands war ends.

"I was elected for five years with a good majority," she insisted. "I intend to serve for five years and then run on my entire record. People know what I stand for and will judge me on that."

Like much of what she says about "honest money," "duties to society" and "standing up for what one believes in," this can easily sound naive or disingenuous outside her constituency of middle and skilled working class Britions who, like Thatcher, have grown disillusioned with postwar welfare-state life and values here.

Thatcher clearly found it frustrating at dinner this week with American correspondents here, their numbers swelled by the Falklands crisis, when she felt unable to persuade many of them that she meant what she was saying about fighting the Falklands war to the end for the principles of freedom, selfdetermination and democracy, and about not following a popular victory with an early election. "What you see," an aide said of Thatcher, "is all that is there."

In yesterday's interview, her first with an American newspaper in three years, Thatcher repeatedly tried to explain why so much of the British public appears to support so potentially costly a war for the sake of 1,800 Falkland Islanders and why she believed this was so significant for the Britain she sees herself as leading.

"I think what we are seeing now is something quite fundamental in the drawing together of the British people once liberty and justice are challenged once again," Thatcher said, summoning memories of the embattled island nation during World War II.

"If you ask a person here what he would associate with Britain, it's not this talk about the welfare state or any sort of benefits or jargon," she said. "He would say, 'We are a free country.'"

Thatcher's economic philosophy is founded on similar beliefs in freedom from government interference or protection, beyond the bare necessities of law enforcement and public infrastructure. She harked back to the Victorian era's unfettered free enterprise when industrialists dipped charitably and voluntarily into its profits to provide for the public good.

"Look at the enormous increase in industry and commerce in this country during Victorian times, which brought with it a consciousness of duty to others," she said. "They built the hospitals. They built the schools. They built the prisons. They built the industries. They built the town halls. They had confidence in the future, and their success brought them the wealth and resources to build the future."

In present-day Britain, Thatcher said, "What I can't stand are all the people who are prepared to go tap the industries making profits for the arts or music or charity, and then, in the next breath, despise industry." Instead, she said, she is trying to attract new interest, talent and investment to British industry so it can help rebuild the country and pay for what is to remain of the welfare state.

Thatcher cited, as a model for the Britain she is seeking to build, "the whole climate and atmosphere in the United States of success. You're expected to be successful and you're applauded when you are. We're trying to get that kind of atmosphere over here, and we shall do it."

She said she is seeking to "build a strong, responsible people and a happy people not a pleasure-seeking people in any way, but a people whose whole talents and abilities are being used.

"Look at a day when you are supremely satisfied just at the end of the day," Thatcher said. "It's not a day when you lounge around doing nothing. It's when you've had everything to do, a real challenge, and you've done it. Life really isn't just an existence. It's using all the talents with which you were born. You can only do that if you've got a government that believes that is the purpose of life as well."

Thatcher acknowledged that this goes against the postwar drift of British society toward greater leisure despite a declining economy. But it matches her own workaholic life style. She boasts of needing less than five hours of sleep at night, has no hobbies and sets aside very little leisure time.

Thatcher's combativeness and headstrong leadership style also is a departure from tacit postwar bipartisan cooperation in running the welfare state through generally collegial Conservative or Labor governments. Although she has been more ready than she publicly admits to compromise with less radical Cabinet collegues on some aspects of economic and other policies, she usually drags the government back to her chosen course through dogged argument often criticized as a schoolteacher's lecturing and even humiliating hectoring of some colleagues.

She has remained convinced throughout the Falklands crisis, for example, that Britain had little alternative but to recapture the islands militarily and restore British administration as it had been before the Argentine invasion on April 2. Under both domestic and international pressure while British forces prepared for their counterinvasion, she reluctantly agreed to compromises that were never, as she expected, accepted by Argentina. Since the big British landing at San Carlos on East Falkland, she has largely shaped events her way once again.

Thatcher insisted in yesterday's interview that "we've never had a more united Cabinet than over the whole Falklands issue." But tacitly acknowledging her practice of working primarily through Cabinet committees of largely trusted membership, Thatcher noted that her inner "war cabinet" contains some of her closest colleagues.

"We've been together a long time," she said of William Whitelaw, the deputy prime minister and home secretary. She added that Defense Secretary John Nott "and I have thought the same way for years."

She denied persistent reports that the war cabinet has been split by differences between her and Foreign Secretary Francis Pym over how much Britain should have been prepared to compromise to achieve a negotiated settlement with Argentina. "He has done absolutely marelously," Thatcher said of Pym, "after being thrown into the deep end" when Lord Carrington resigned after the Argentine invasion of the Falklands.

"They're all in my camp," Thatcher said. But she added that, "of course, in formulating policy we argue. What else would you expect? When you're talking tactics and what to do next, you've got to talk things through. You owe that to the British troops.

She insisted that she does not bully or ignore her Cabinet on this or other issues, but rather that she works out decisions through sometimes vigorous argument, which she suggested some people might misunderstand. Thatcher's love of verbal combat is evident at prime minister's question time in Parliament and in press conferences. Her eyes light up and she smiles broadly when she believes she has scored a point, oblivious to the possibility that her style sometimes may appear needlessly abrasive.

"The idea that a prime minister is there just as a chairman to collect votes is absolute nonsense," Thatcher said of her approach to Cabinet government. "You're there to give a lead. You may modify your approach as a result of discussion."

On the other hand, she added, "you must never have a head of government who will not permit argument. A lot of people work like myself, and you have to argue a thing through. If you have a view, you must be prepared to put it to the test and submit it to argument.

"I don't want to be the sort of person who has toes to tread on, if you understand what I mean," Thatcher said. Among her oldest colleagues, she said, "We just have this saying, 'We don't have any toes.' We do have toes, of course. But we are not so concerned about someone else trodding on our toes that it dominates our views for heaven knows how long. You just carry on. What matters is that we get the right answer between us."