London — So large is Prime Minister Margaret Thatcher's lead in the polls here that the main question is less and less whether the Conservative government will be returned to office next Thursday.
Rather, it is what Britain's political, economic, and social landscape will be like after her expected victory.
Much will depend, of course, on just how large a Tory victory margin turns out to be. Analysts, including Prof. Ivan Crewe of Essex University, have been translating the huge Tory lead in the polls (between 12 and 22 percent) into a majority of more than 100 seats in a House of Commons newly enlarged from 635 to 650 seats.
If so, the results will be hailed by the Reagan administration and NATO. They will be gratifying for the prime minister and her close supporters, shattering for the opposition Labour Party and its leaders Michael Foot and Denis Healey, and problematic for the Social Democratic Party (SDP) and Liberal alliance.
Meanwhile little will change on other fronts.
Britain will still be divided into a largely out-of-work north and a comparatively prosperous south. The 12.7 percent unemployment, already at record levels, will keep on rising for the near future at least, even government ministers admit. British manufacturing industries will remain hard-hit by recession.
Economic recovery will remain patchy. The latest survey of 1,800 companies by the Confederation of British Industry reports a degree of rising optimism but hedges its forecasts with caution.
Mrs. Thatcher's personal preeminence, based in part on her victory last year in the Falklands war, will probably rise still higher.
To the satisfaction of the Reagan administration, Britain will remain firmly pro-US, pro-NATO, pro-European Community, and sternly anti-Soviet.
US bases here will stay. Cruise missiles will be installed in concrete bunkers at Greenham Common air base. Britain will go ahead with buying the US Trident submarine missile, though many defense analysts think the cost - between (STR)8 billion and (STR)10 billion (about $13 billion and $16 billion) - will ultimately prove too high.
Politically, the next Thatcher Cabinet is likely to see quick changes. So-called ''dry'' Tories who support Mrs. Thatcher's monetarist attacks on inflation will almost certainly be promoted. ''Wets,'' moderate Tories who want more spending to ease joblessness and are widely seen as upholding the Conservative ''conscience,'' will likely be eased out.
Among the ''wets'' headed for the door, it is said, are such prominent party stalwarts as Foreign Secretary Francis Pym, Home Secretary William Whitelaw, and Northern Ireland Secretary James Prior. Political insiders here claim that the chancellor of the Exchequer, Sir Geoffrey Howe, will replace Mr. Pym, and that Employment Secretary Norman Tebbit, a prime ministerial favorite, will take over the Home Office.
Mrs. Thatcher will continue to hammer away at holding down inflation (it is standing at 4 percent), saying that it is the only genuine, lasting method of reducing unemployment in the long haul.
Meanwhile, massive unemployment benefits will continue, as will plans to spend more than (STR)1 billion ($1.6 billion) on a new Youth Training Scheme aimed at giving every 16- and 17-year-old school-leaver in the country who cannot find work one to two years of training and study. Today 1.3 million of Britain's more than 3 million unemployed are under 25, government figures show, and the figure will grow when school-leavers begin looking for jobs this summer.
One forecaster, the Cambridge Economic Policy Group, predicts 4.2 million registered unemployed by 1988. Another group, Cambridge Econometrics, predicts just over 4 million.
The opposition Labour Party keeps predicting that Mrs. Thatcher will weaken the National Health Service as part of a ''secret plan'' to dilute the welfare state. Mrs. Thatcher denies it.
At the same time, Labour's own future seems sure to be intensely difficult. Even if it does better than its current standing in the polls - about 20 points behind the Tories, appealing to only about one-third of those polled - the next few months will see a backstage fight to depose leader Michael Foot.
At the Labour Party's annual conference in the fall, radical left-wing constituency parties and some trade unions will try again to install a far-left leader such as Tony Benn.
Many analysts believe that Mr. Healey is too far to the right of the party to win conference support. Tony Benn may prove too far to the left of the party. Other main contenders include:
1. Neil Kinnock, who would give up British nuclear weapons, would leave the European Community, and supports a stronger welfare state. (The Labour Party platform calls for a reduction in dependence on nuclear weapons and an eventual non-nuclear defense, withdrawal from the EC, and increased spending on social programs.)
2. Peter Shore, shadow chancellor, who is skeptical of the party's support for nuclear disarmament but goes along with its opposition to British membership in the EC.
3. Roy Hattersley, shadow home secretary, who has reservations about the party's policies on both defense and Europe.
The best that the SDP-Liberal alliance can hope for, as SDP founder David Owen concedes, is to hold the balance of power in a hung Parliament. Even this looks unlikely at this writing - although the latest polls indicate that the alliance has made some modest gains at the expense of the Conservatives.
The weekly magazine Economist estimates that, under Britain's winner-take-all system, if the alliance share of the national vote stays between 20 and 25 percent, only between 4 and 17 Liberals would be returned to the House of Commons - and the SDP would win even fewer seats. It would require a 35 percent share of the vote for the alliance to make real gains in actual parliamentary representation, the weekly says.
A poor showing would mean pressure on SDP leader Roy Jenkins to yield his spot to Dr. Owen or SDP president Shirley Williams, and increased prominence for Liberal leader David Steel.