London — ''We're digging in our heels for the long haul,'' says one prominent peace campaigner about United States cruise missiles coming to Britain. In other words, the British peace movement, one of the largest and most active in Western Europe, is on the defensive following a series of reverses.
Among the setbacks, even as the end-of-year deadline for the first deployment of cruises draws closer:
* The reelection of pro-cruise-missile Margaret Thatcher with a large majority in the House of Commons; the election of pro-missile (both the cruise and Pershing II) Chancellor Helmut Kohl in West Germany; less-than-expected crowds at recent peace protests in Britain.
* The Greenham Common women peace protesters receiving less publicity while the Thatcher government's campaign against them has made headway.
* And British public opinion polls suggesting a rising number of people willing to accept US medium-range missiles in Britain - together with a majority in one poll for taking them if disarmament talks with the Soviets fail in Geneva.
Mrs. Thatcher's reelection makes it even more certain that, barring progress in the Geneva medium-range missile talks, the first cruise missiles to come to Europe will be installed at the US Air Force base at Greenham Common near Newbury in Berkshire, before December.
In interviews, peace group spokesmen here put the bravest face they can on the election results and the post-election period.
''It's certainly not time to pack up and go home,'' said Jane Diblin of European Nuclear Disarmament (END).
''There's been some disillusionment. It's a time for stock-taking. . . . We don't see the election as a vote against our policies. People voted against Labour and for firm leadership. . . . People still ask why unilateral disarmament is necessary. . . . Our point has not been put across clearly enough yet.''
''Nothing has changed,'' insists Alison Whyte of the largest peace group, the Campaign for Nuclear Disarmament (CND).
''CND has thrived during the first four years of Conservative rule and we will continue to do so through the next four or five years. . . .
''We have more than 55,000 national members now, and we're still gaining them at the rate of 1,500 per week.''
She referred to a Marplan poll in the Guardian newspaper in May which she said showed that 60 percent of British people opposed cruise missile deployment here.
At Marplan headquarters, Nick Sparrow said that in fact, 54 percent had said they disapproved the British decision to allow the US to base cruises on British soil.
Thirty-four percent (of 1,422 polled) said they approved. Twelve percent didn't know.
On the face of it, CND can claim that a majority of people polled don't want the missiles.
But the issue is more complicated.
An earlier and similar Marplan poll, in January, showed 61 percent disapproving and 27 percent approving - which means that those in favor of the cruise are increasing.
Also, Tim Burns of another respected polling organization, Market Opinion and Research International (MORI), said that the figures change again when people are asked if they would accept the missiles if the Geneva talks failed.
In a poll in May, 52 percent said that under those conditions, they would say ''yes'' to cruise. Thirty-four percent said they would not. This was a marked shift from January, when in answer to a similar question, only 36 percent had said they would accept cruise and 54 percent were opposed.
''What we're seeing,'' said Mr. Burns, ''is that a large segment of British opinion favors the missiles, another large section doesn't, and there's a group of 'don't knows' in between.''
What do the peace groups in Britain do now?
CND is devoting its next quarterly executive committee meeting to strategy. It remains committed to ''considered, nonviolent direct action'' in the words of one spokesman, such as the ''peaceful'' blockading of the gates to US Air Force bases.
Meanwhile, previous methods continue. Scattered protests are planned for US Independence Day July 4, including a CND ''Declaration of Independence'' to be handed in at the US embassy by CND's Monsignor Bruce Kent.
On July 16 the London CND branch is organizing a ''human chain'' of demonstrators from the Soviet embassy to the US embassy.
A large march is planned through London October 22.
Activists concede that much more is needed to reignite the movement and get back to the enthusiasm and publicity of six months ago.