By all accounts, when Margaret Thatcher, then leader of the Conservative opposition here, last was in Washington two years ago, she was not taken terribly seriously. In contrast to the experience and assurance of then British Prime Minister James Callaghan, she seemed ill at ease, poorly informed and not ready for the world stage.

A decidely different Margaret Thatcher arrived in Washington today. She will meet with President Carter at the White House Monday to confer on Iran, nuclear weapons, the world economy, energy and other important issues. He will find her brimming with self-confidence, overflowing with facts on every subject, and even freer with advice than Callaghan used to be.

Thatcher will point to the progress her government has made in trying to negotiate a peace agreement in Rhodesia and the plan it has for seeking political accord between the Protestant majority and Catholic minority in Northern Ireland.

She will offer strong views on how the Western democracies must strengthen their military and diplomatic resolve against the Soviet Union. She will explain why she wants Britain to maintain its own independent nuclear deterrent, in addition to the NATO nuclear missiles deployed at U.S. bases here, and may seek to modernize tht deterrent with new U.S. nuclear submarines and missiles.

But in economically troubled Britain, Thatcher is entering what promises to be a difficult testing period for her fiscal policies of tight money and government spending cuts, which so far have failed policies of tight money and rejuvenate private enterprise. But from her point of view, she is still riding high seven months after becoming prime minister.

"I like this job," she enthusiastically tells anyone who asks, and she devotes nearly all her time to it. She pores over paperwork until the early morning hours almost every day and most weekends. She briefs herself to the last detail on every issue and likes to spout statistics, toss out expect terms and dramatically demonstrate her resolve once she has mastered a subject and charted her course.

Energy is one of her newer passions. When a reporter questioned her about British energy policy at the recent Common Market summit in Dublin, Thatcher asked him, "Are you an expert? Do I have to give you the long explanation or a short one?"

Officials in Washington and business figures in New York will see the confidently determined Thatcher who lectured other European leaders at the Dublin summit on Britain's "very just and equitable case" for easing what she believes is its disproportionate share of the funding of the Eurpoean Economic Community. "We want our money back," she repeatedly demanded.

When she returned here she reported she would give those EEC nations who were "genuine friends" of Britain "one last chance" at the next Common Market summit to refund most of the $2 billion in British contributions she wants back. Callaghan, now the opposition leader, criticized Thatcher's undiplomatic approach, which had alienated many of the other European leaders.

"I think that you really cannot talk to the President of France or [West German] Chancellor [Helmut] Schmidt as though they ar mentally deficient," Callahan said. "They have a mental age of rather greater than 7, you know."

Thatcher does not apologize for her sometimes strident nationalism or the aggressive way she pursues what she believes is right. She says she cannot bear to be described as pragmatic "because that means you don't believe in anything." She delights in being seen as controversial and as a latter-day image of the late French President Charles de Gaulle who cannot be taken for granted.

As she will tell Carter, her vision of Europe is not that of a federation, or even confederation of member states gradually losing their national identy within a Common Market that has designs on being more than a club for free trade.

Rather, she believes Britain should remain in the EEC, despite its present economic drawbacks for a less competitive Britian, to strengthen Europe's industrial and military cooperation against the Soviet threat. Each member state, in her view, would retain individual national sovereignty and prerogatives while acting collectively as a bulwark against the Soviets.

She says that the 34 democracies in the world must stick together to avoid being picked off one by one. She was pleased when the Soviet media accused her of trying to "wear Churchill's pants" in adopting a tough anti-Soviet line reminiscent of the Cold War era. She believes that Britain, because of France's coolness to NATO and West Germany's proximity to the Soviet Union, should again be Europe's most outspoken defender of freedom.

She also wants to invigorate Britain's sick economy by shrinking the government's role through spending cuts and the lifting of controls and encouraging business incentives with income tax cuts and "management freedom to manage again."

She has pointed out that for the first time since World War II, there are no government controls here on pay, prices, dividends or foreign exchange. But others in her own government are worrying aloud that prices and wages are still rising dangerously, while Thatcher's strict monetarism may deepen next year's predicted recession and greatly increase unemployment.

Britain's inflation rate rose above 17 percent this month, matching the record high interest rate. The mortgage interest rate ranges from 15 to 19 percent, which greatly increases mortigage payments for middle-class Britains, who were an important element in the Conservatives' election victory. They also are facing big increases in television license fees, commuter fares and fuel prices.

Public opinion polls show that Thatcher's Conservatives trail the opposition Labor Party by four percentage points. Many women and skilled workers who had voted Conservative for the first time in last May's national election have since turned away from Thatcher.

Thatcher acknowleges that her economic policies likely will not produce encouraging results until two years from now. A growing number of senior Conservative members of Parliament, including some in Thatcher's cabinet, fear the government will suffer too great a loss in public support in that time.

Many also fear adverse reaction to some of the right-wing social policies Thatcher has pursued, including limiting immigration in a way that discriminates against both nonwhites and women, forcing parents in rural areas to pay for school transportation for the first time, increasing the National Health Service fees for medicine and dentistry, and raising public housing rents.

These critics still talk behind Thatcher's back about the possibility that she might be replaced as party leader and prime minister in midterm or be forced to drastically change some policies.

But Thatcher remains determined to do things her way as long as the Conservatives have a comfortable majority in Parliament. Their current 43-seat majority is large enough to withstand a large number of by-election defeats as parliamentary vacancies occur before the next national election in 1984.

Thatcher, who has few consistently close advisers and no "kitchen cabinet," intends to continue with her intensely personal, sometimes abrasive style of leadership, trying to oversee every detail of government like a fussy schoolmistress.

On her way through the front hall of 10 Downing Street last week, she paused in front of the newly installed, still untrimmed Christmas tree. "Is it going to be decorated now?" she asked a startled aide as she hurried over to inspect the tree closely. "That branch isn't going to poke into the picture on the wall, is it? Then she rushed off to her waiting car and the next task awaiting her.