London — Nuclear unilateralism is on the march in Britain and may well be part of the next British government's defense policy. The Campaign for Nuclear Disarmament (CND), is once again a spearhead of antinuclear protest in Britain, after a quiescent period during the 1960s and ' 70s. In two years its national membership has grown from about 3,000 to more than 30,000.
Two infant organizations, European Nuclear Disarmament, advocating, like CND, a European nuclear-free zone, and World Disarmament Campaign, have also attracted growing support.
On Oct. 23, the day before a record crowd of 150,000 demonstrated in London against nuclear weapons, a unilateralist, Dr. Nicholas Humphrey, gave the prestigious BBC Bronowski Memorial Lecture on television. The BBC Radio Times noted that what had been a sentimental heresy in the 1960s was ''fast becoming political orthodoxy from left to center.''
Labour Party leader Michael Foot, in theory Britain's next prime minister, should Margaret Thatcher's Conservatives be voted out, said at the London rally that the world was facing ''its most critical moment in the nuclear arms race.''
Some weeks before, he had come out uncompromisingly unilateralist after votes in that direction at the Trades Union Congress and Labour Party conferences.
Saying he was ''starkly'' critical of the effect of recent American policy on the Atlantic alliance, he said he had ''not the slightest doubt'' that the next Labour government's policy would have ''strong strands of unilateralism.''
That was long before President Reagan's Oct. 16 remark that it might be possible to limit an initial nuclear exchange to Europe touched off such a furor among the Western allies.
So long as Mrs. Thatcher is prime minister, Britain will remain a nuclear power, willing to accept cruise and Pershing II missiles and to equip its submarines with Trident missiles. But these weapons will go if Mr. Foot takes over.
No one knows what would happen if anyone other than Labour formed the next government.
Liberals, already against Trident and an extension of Polaris, voted this year against cruise and Pershing II missiles and the neutron bomb and for a European nuclear-free zone.
The Social Democrats have not yet voted on defense - or on anything else.
Their foreign affairs spokesman, Dr. David Owen, hopes cruise and Pershing II missiles won't be necessary but doesn't want to foreclose the option.
He seems to accuse Labour of choosing unilateralism for the wrong reason: because it is popular. It is a revealing admission.
If his own party really believes that the people should have what they want, it could be that the Social Democrats, reflecting growing public opinion and making up their own minds, will eventually come down, with Labour, on the side of unilateralism.