MARGARET THATCHER is about to exchange 32-years in the House of Commons for a role promoting democracy and free enterprise in countries casting off their legacy of communist dictatorships.The "Iron Lady" of British politics made this plain after her decision June 28 to give up her Commons seat at the next general election, enter the House of Lords, and throw her energies into leading her planned Thatcher Foundation. She told journalists June 29 that she wants to focus on helping the new democracies of Eastern Europe, where the foundation will have "branch offices." "I intend to nurture democratic values in the former communist states," Mrs. Thatcher said. "My aim will be to perpetuate all the kinds of things I believe in. They know what they want to do. They don't know how to do it." The swift move to a new calling seemed typical of the dynamic politician who, in 1979, became Britain's first woman prime minister, and who dominated British politics for nearly a dozen years. "Like her or not, her impact has been enormous," says Hugo Young, author of a best-selling Thatcher biography. He described her decision to step down from Parliament as "the end of an era." The grocer's daughter who gave her name to a conservative political and economic philosophy, and played a prominent part in opening Western contacts with President Mikhail Gorbachev, appears determined to help shape the post-communist era. When she enters the House of Lords after the next election, the upper chamber will provide her with a parliamentary platform for political intervention. Voted out of office last November and replaced by John Major, the former prime minister has served notice that she is determined to remain vocal in the debate about Britain's future in Europe. Two days before saying she would resign her parliamentary seat, she stunned the House of Commons with a speech condemning European federalism. Afterward, Prime Minister Major said he accepted her right to speak. Mr. Major's supporters, however, said he was bothered. "She seems to be saying that John will get her support only if he follows her policies, and that she will criticize if he does not," said one Member of Parliament. "That is a recipe for trouble." In the seven months since Major was chosen to replace Thatcher, there has been relatively little assessment of the long-term impact she has had on Britain. The spotlight immediately fell on the new man at 10 Downing Street. Her declared intention to leave the Commons, however, has unleashed a flood of evaluations. John Grigg, a veteran historian and political commentator, says Thatcher "restored Britain's reputation in the world by her ability to say 'no' to disruptive forces at home and by the firmness of her stand against Soviet tyranny." Robin Oakley, political editor of the London Times, said that for Major and later prime ministers, Thatcher "will be the yardstick." "Whatever their separate qualities, every politician after her will seem like a man or woman of compromises," Mr. Oakley said. About her political philosophy, views clash head-on. Nicholas Ridley, a former senior Thatcher minister, underlined her impact on socialism. "She forced the Labor Party to abandon, one by one, the basic policies of socialism," he said. "She forced it to abandon unilateral nuclear disarmament." Neil Kinnock, leader of the Labor opposition, assessed Thatcher's policies as "extremely destructive for Britain's future ... I will not miss her." Peter Jenkins, a columnist for the Independent newspaper, and author of an authoritative book, "The Thatcher Years," says that Thatcherism is less a well-defined set of policies, and more accurately "a style, a bundle of simple principles or prejudices laced with remarkable, single-minded determination." Mr. Young, the biographer, says Thatcher's impact on the debate about Britain's future in Europe may not be great. "I am skeptical about the feeling that this great empress, as she was, still commands any enormous support," he says. "I think she will speak more, but with less and less influence." Grigg makes a distinction between the Thatcher legend and the person. "Thatcherism may be a myth, but Thatcher herself is a genuine and potent reality." Outlining her plans for the Thatcher Foundation, she said it would arrange conferences from its London headquarters, make available education grants, commission research, and provide help to East European countries and the Soviet Union. "A chain of foundation offices will act as contact points," she said. If her past work habits are any guide, Thatcher will devote 18-hour days to making certain that her philosophy is conveyed to all who seek guidance, and even to those who do not. Summing up the gains she made by deciding to leave the House of Commons, she exclaimed: "I'm free, I'm free!"