LONDON -- After Margaret Thatcher's decisive third-term victory, there is wide agreement here that Labor's demand for unilateral nuclear disarmament not only lost the June 11 election but bars it from power far into the future.

Excited Conservatives are now fantasizing on Prime Minister Thatcher or a hand-picked successor's holding power into the mid-1990s. ''Neil Kinnock has managed to lose not just one but two elections,'' a senior Labor M.P. told us. Calmer heads than politicians agree. A London bookie set odds favoring a Tory victory in the next general election at 7 to 2.

That gives a salutary lesson to Britain's beleaguered Labor Party and to the democratic left everywhere. Labor posed an alternative to the Conservatives unacceptable to British voters, who want a nuclear deterrent against the Soviet Union.

Kinnock's unilateralism is not Labor's only problem. London's ''Loony Left'' elected new members to Parliament. Rising prosperity coincides with revived energy in the Thatcher government, which belatedly will cut income tax rates.

But the hugeness of Labor's loss was disguised by Kinnock's charisma and his campaign's technical proficiency. Despite minor gains (concentrated in Scotland), Labor remains 147 seats behind the Conservatives -- requiring an unprecedented leap forward next time to capture the government.

What's more, Labor in 1987 enjoyed assets unlikely to be duplicated. A relatively unknown Kinnock soared past Thatcher in approval polls after a brilliant biographical commercial on television (produced by Hugh Hudson of ''Chariots of Fire'' fame). That effect cannot be repeated. Nor is it likely the Socialists would outdo the Tories in technical proficiency two campaigns in a row.

Neither can Labor expect a slump from the prime minister. She does not enter her third term, as Ronald Reagan did his second, with an impoverished agenda. She may not be bold enough to suit the right in dismantling the welfare state, but she will be more radical in her third term than her second. That means pursuing privatization and at long last bringing Britain's growth-depressing top tax rate below 50 percent.

None of this gives hope to the Socialists. But even long-range aspirations will stay dim if nuclear unilateralism persists as Labor policy. That is the private view of a substantial minority of the parliamentary party and perhaps one-third of the shadow Cabinet. The notion that defense means nothing to any Briton born after Dunkirk was exploded by the June 11 vote, cutting across age (and class) lines.

Moderate labor unions, who pay the party's bill, want a change. Union leaders are conferring with John Gilbert, a Labor back-bench defense and arms control expert who has beena lonely figure in his party, preach-ing against unilateralist idiocy. Although he gagged himself during the recent campaign, he is talking sense again.

But Roy Hattersley, Labor's moderate deputy leader, has not renewed his post-1983 election stand against unilateral disarmament. After this election, he said abandonment of the Kinnock position would do ''almost irrevocable'' damage to the party. Indeed, able and combative doctrinaire leftists are included in newcomers to Labor ranks in the House of Commons.

But the real problem is Neal Kinnock. He has copied Harold Wilson's pragmatism on most issues in moderating his stance after winning party leadership as candidate of the left. But not on nuclear arms. If Labor wants to change defense policy, it must sack the leader who just restored its confidence. It cannot have Kinnock without keeping unilateralism.

That is a delicious dilemma for the Tories to enjoy, but not so good for the two-party system. A loyal opposition lacking credibility in defending the nation from foreign threats is not politically viable. That is Britain's lesson for the democratic left worldwide, but there is no certainty it has yet been learned here.