Britain's opposition Labor Party appears to have strengthened itself as an alternative to Prime Minister Margaret Thatcher's Conservatives by significantly reducing the influence of its militantly socialist left wing at the party's annual conference here this week.

The party accomplished this, against the expectations of many, by reelecting rightist Denis Healey as deputy leader over left-wing standard bearer Tony Benn and consolidating that victory by regaining from Benn's supporters control of the powerful, policy-making National Executive Committee.

The party leadership also turned back efforts to make Labor members of Parliament subservient to the letter of party policy and local constituency groups, which largely are controlled by leftist activists.

At the same time as the campaign against the left, support has grown within Labor's constituency groups and sponsoring labor unions to ban nuclear weapons from Britain, a stand encouraged by party leader Michael Foot. After an emotional appeal by Foot yesterday for worldwide arms reductions, the party conference today voted overwhelmingly to reaffirm its commitment to unilateral nuclear disarmament and reduced defense spending.

The actions against the left of the party are expected to slow defections to the new Social Democratic Party, founded earlier this year by politicians who broke away from Labor because of the growing influence of militant leftists. A number of Labor members of Parliament had indicated that they would join 15 former Labor deputies who are now Social Democrats if the march of the left were not stopped at this year's party conference.

A slowdown or cessation of defections from Labor could be a serious setback for the recently formed alliance of the Social Democrats and the small Liberal Party, which had risen rapidly in public opinion polls as a centrist alternative to the political polarization of Thatcher's right-wing Conservative government and Labor's shift to the left. Opinion polls have shown that many more voters might desert Labor if the left captured the party leadership.

Instead, Healey and his followers on the party's right wing, backed by former prime minister James Callaghan and a number of trade union leaders commanding large bloc votes in the Labor Party conference, have succeeded in a hastily organized campaign to fight back against the leftist insurgents. They were joined by leftists loyal to Foot who agree with the militants on most issues but object to attempts to limit the freedom of Labor members of Parliament or future Labor governments.

The party leadership is now freer to modify or ignore radically socialist policies the left has succeeded in having enacted by party conferences in recent years, including greatly increased nationalization of industry and prohibition of private education and health care. But both Foot and a majority of the party's new executive committee remain committed to two other controversial policies: withdrawing Britain from the Common Market and banning nuclear weapons from British soil.

By a vote just short of the two-thirds majority required to guarantee its inclusion in Labor's next election platform, the party conference today approved a resolution reaffirming its opposition to replacement of Britain's aging Polaris submarine nuclear deterrent and to "the deployment of cruise missiles, the neutron bomb and all nuclear weapons in or by Britain." The party conference also reaffirmed its commitment "to close down all nuclear bases, British or American, on British soil or in Britain's waters as Britain's direct contribution to the creation of a European nuclear weapons-free zone and as a powerful British initiative in the wider process of nuclear disarmament."

This policy is opposed by Healey and several other former Labor Cabinet members and Callaghan as unrealistic and incompatible with the party's support for British membership in the NATO alliance, which also was reaffirmed today by a 3-to-1 vote.

Healey and his supporters have argued instead for a policy vigorously putting pressure on the United States and the Soviet Union to negotiate arms reductions that could eventually remove nuclear weapons from Britain.

However, Healey has joined Foot in opposing replacement of Britain's Polaris nuclear deterrent with an expensive, American-designed Trident submarine-launched missile system and has spoken against the Reagan administration's decision to produce the neutron weapon.

The Labor Party remains ideologically splintered in other ways, and Benn's left-wing supporters have vowed at well-attended fringe meetings here to continue their aggressive grass-roots organizing in local constituencies and labor unions in an attempt to undo this week's setbacks. Benn himself has not yet indicated whether he will challenge Foot or Healey again for the party leadership.

A tireless campaigner and persuasive public speaker, Benn, 56, remains a very popular figure among party activists. But he is unpopular with many voters, according to opinion polls.

Healey, 64, a burly man with dark bushy hair and eyebrows, is much more trusted than Benn by voters, according to opinion polls. But he is unpopular within the party structure because of his abrasive manner and decisions he made as chancellor of the Exchequer in the last Labor government.

Foot, 68, an eccentric intellectual, political maverick and veteran disarmament campaigner, comes closest to bridging Labor's ideological divide and commands the widest popular support. Until this week, he had appeared to be a weak leader of a feuding party, although he may now be more in command because of the decisions of the party conference.