Prime Minister Margaret Thatcher's landslide victory is forcing a major realignment of leadership in British politics, bringing a new generation further to the fore in all the country's parties.

Policy-making and political strategy will be influenced more than ever before by people who were still children at the end of World War II and who have not been primarily responsible, as one analyst put it this week, for "the orderly management of Britain's decline."

While by no means newcomers to prominence, their perspectives on Britain and the international scene have been shaped by a lifetime that began as much as decades after that of their predecessors. They are the first British leaders to mature since the empire was lost. That should give them an edge in defining Britain's new world role.

The resignations of Labor Party leader Michael Foot and Roy Jenkins, leader of the Social Democratic Party, remove two men who have been among the most important and active in British public life for 30 years. Thatcher's ouster of foreign secretary Francis Pym consigns to the political wilderness the person regarded only a year or so ago as her main challenger for leadership in the Conservative Party.

And her promotion of home affairs minister William Whitelaw to the House of Lords ends his long career in the rough and tumble of the House of Commons.

In their places at the front ranks of the parties are coming names like Neil Kinnock of Labor, David Owen of the Social Democrats and Leon Brittain, the new home secretary. All are under 45, or almost 25 years younger than Foot, 20 years junior to Whitelaw and more than 15 years younger than either Jenkins or Pym.

Already in place is Liberal Party leader David Steel, 45, who has led his party since 1976. Steel added to the current atmosphere of political volatility by suggesting in a radio interview today that he might step aside before the next general election, after serving 10 years as party chief.

Such a move--and the immediate reaction from Steel supporters was that he was probably weary from the campaign--would be a serious blow to the Liberals. The party's success in gaining a substantial popular vote was attributed to his comparatively vigorous and youthful image in contrast to Jenkins and Foot.

The issue was not chiefly their chronological age, but what seemed in both cases to be an outdated manner and political style.

The significance of this inevitable passing of the torch goes well beyond Britain. Throughout Europe what is called by political scientists "the successor generation" is gradually coming to power. These are men and women who have no first-hand memories of the 1930s economic depressions, the wartime collaboration with the United States and the dangers posed by Stalinist communism in the early postwar years.

They are, as a group, the first generation for whom television is a completely familiar instrument (as computers will be for those that follow them in 15 or 20 years). One of the features that binds, for instance, Owen and Kinnock, a duo whose political beliefs differ widely, is that they use a televised platform with great skill. Foot and Jenkins never could master the medium and it was consistently held against them.

The exact meaning of this transition in terms of Britain's future is hard to judge. But there is little doubt that the atmosphere of politics and policy will be altered in time, as it was in the United States 23 years ago when John F. Kennedy, at 43, became the first president to be born in the 20th century.

Thatcher, herself, of course, does not come from that younger pool. She is 57 and in her revamped Cabinet, many of the top jobs belong to men who are her contemporaries, or slightly younger, in their early fifties. In making those postelection choices, however, she alone among British party leaders was acting from strength, as Peter Jenkins, political editor of The Guardian observed in an interview today.

Labor and the Social Democrats, the two parties who were least successful in the campaign, moved swiftly to revamp their fortunes by shedding their most visible liabilities, Foot and Roy Jenkins. Kinnock is the undisputed front-runner for Foot's job, although he faces competition for the post from Roy Hattersley, 50.

Owen, who almost certainly will not be challenged for the Social Democratic leadership, achieved a distinction by becoming Britain's youngest foreign secretary at 38. But neither Labor, which he then served, nor the Social Democratic Party, which he helped form in 1981, was initially inclined to follow his lead. His brashness and what was considered his unattractive ambition have apparently come to be regarded as just the dynamism that the party needs.

It is certain, therefore, that by next fall, Thatcher will be facing three opposition party leaders across the parliamentary aisle who are not, in the British phrase "yesterday's men."

At the least, this should provide the country's political climate with a healthful gust of fresh air.