Mrs. Thatcher consolidates victory with cabinet changes

Britain moves into the mid-1980s with rising influence in Washington, with an expanding set of middle-class values at home - but with its own political, economic, and social fabric still in need of urgent repair.

These are the main conclusions being drawn from Prime Minister Margaret Thatcher's overwhelming election victory, followed by a quick Cabinet reshuffle promoting younger loyalists to her inner circle.

The increased Tory majority over all other parties - 144 seats in a 650-seat House of Commons - reflects support for Mrs. Thatcher's leadership qualities and enhances her already dominant position in the country. But a number of analysts here point out that it does not fully mirror the actual vote.

The Tories share of the total vote dropped to 42.4 percent from 43.9 percent in 1979. However, the British ''first past the post'' system of voting turned Mrs. Thatcher's vote into a large representation of 397 seats, enhanced by the Labour Party's dismal showing.

Labour slid to 27.6 percent of the total vote, down from 36.9 percent four years ago, giving it 209 seats. The Social Democratic-Liberal alliance attracted almost as many votes - 7.7 million and 24.6 percent of the vote, yet under the British system could obtain only 23 seats.

Meanwhile, opposition political parties here launch into a period of difficult reappraisal with the aim of contesting the center ground of politics more penetratingly.

Effective political opposition within the House of Commons moves from Labour and the Social Democratic-Liberal alliance to the moderate, or ''wet'' wing of the Tory party itself. Its task is formidable.

Mrs. Thatcher asked voters for a big majority in part, as she said repeatedly , to enhance Britain's role in the world. By so doing she touches a wellspring of patriotic, days-of-empire yearning deep in many British people who are upset by Britain's decline abroad.

In foreign policy, Mrs. Thatcher has almost single-handedly revived the ''special relationship'' with the United States. The prime minister now goes into international meetings with the added authority of a leader massively re-elected and in office for a longer continuous period than any of her major allies.

President Reagan is reported to admire Mrs. Thatcher for her toughness and her electoral skill - a number of Republican strategists were in London to study her methods - and for the outspoken support she gives him.

Her new foreign secretary, Sir Geoffrey Howe, is one of her closest supporters and confidants, and is expected to speak for her with authority. (The prime minister has dismissed Francis Pym with whom she has had numerous disagreements.)

On the home front, although her share of the total vote dropped a little, the prime minister is seems closer to popular aspirations than the Labour Party.

Class differences are wide, but the Britain that re-elected Thatcher increasingly: owns its own home (61 percent), has its own bank account (61 percent of all households), and possesses such amenities as telephones and central heating (three-fifths of all homes). More than 600,000 tenants have been able to buy their council (public) homes under Tory legislation.

The new chancellor of the Exchequer, Nigel Lawson, is a strict monetarist. Norman Tebbit remains as employment minister; he is expected to pilot through the Commons legislation forcing unions to hold secret ballots for their leaders, to hold votes before calling strikes, and to let members choose whether to pay a political levy to the Labour Party.

This will further illustrate the difficulties facing the Labour Party, which simply misjudged the mood of the country as well as, in the view of many commentators, mismanaged its own campaign.

The party failed to reconcile deep differences between its far-left and moderate wings. It alienated the middle ground by proposing to scrap nuclear defenses, to leave the European Community at a time of high unemployment, and to resort to government pump-priming to try to create new jobs.

As a result it lost one-fifth of the votes it gained in 1979. According to a commercial television poll, it even failed to pull in a majority of the two groups upon which it has traditionally counted: the unemployed and trade union members.

Now Labour sets out on the difficult road to reconstruction, as the Democrats in the US had to do in the Eisenhower years and the Republicans in 1960. Ostensibly, the issue is selection of a replacement for Michael Foot, who announced Sunday that he would resign.

An important question is whether radical, younger, college-educated Marxists who control grass-roots constituency parties can tilt the party further to the left - or whether moderate trade unions and members of Parliament can drag it back to the right. Of the three main younger contenders - assuming that former Foreign Secretary Denis Healey cannot make it - Neil Kinnock is further to the left than Roy Hattersley or Peter Shore.

The battleground will now be a series of trade union conferences - iron and steel trades, health workers, railroads, shipbuilders and engineers - starting in June and climaxing with the Labour Party's annual conference in Brighton in the fall.

The Social Democratic Party under Roy Jenkins managed only six seats, and lost two of its founding ''gang of four,'' Shirley Williams and William Rodgers.

The Liberals did better with 17 seats, and leader David Steel performed well on television throughout the campaign.

The alliance now plans to push hard for a national referendum on introducing proportional representation, which allows smaller parties to share power. Mrs. Thatcher is opposed. Failing that, Mr. Steel says he will circulate a national petition.