The leader of Britain's opposition Labor Party, told by Soviet President Konstantin Chernenko in Moscow last week that "the U.S.S.R. would guarantee that its nuclear weapons would not be targeted on British territory" if Britain got rid of. nuclear weapons based here, said Wednesday that in a crisis nobody really knew what would happen.

Labor leader Neil Kinnock, meeting with American correspondents here, spoke about the two key "offers" made by Chernenko contingent on a future Labor government carrying out nuclear disarmament in Britain: pledges not to target nuclear missiles on Britain and to match, missile-for-missile, the British reductions.

The Labor Party, in October, adopted a defense platform that would commit it, should it come to power, to unilateral nuclear disarmament, including closure of U.S. nuclear bases. But the platform confirms a commitment to conventional defense and to membership in NATO.

Kinnock was asked Wednesday how much validity he put in Chernenko's pledge not to aim nuclear missiles at Britain -- if, for example, thousands of allied forces were moving through Britain during a prewar crisis.

"I don't think we are capable of anticipating, on either side, what the reaction would be if the nuclear threshold were crossed," he said. "In that circumstance of crisis, I don't know and nobody else knows what will happen."

When pressed to assess how much credence he put in the Soviet pledge, Kinnock said the pledge is really about a missile "build-down."

"It's not about what would happen in a crisis," he said, but rather about "an initiative that gets a missile-for-missile exchange reduction, and that is a useful step in the right direction." When it was noted that the so-called "build-down process" is a negotiating term used by the United States to achieve mutual missile cutbacks with Moscow, and that the Labor Party policy is to get rid of British weapons unilaterally, Kinnock said that "what we've got . . . far ahead of the assumption of power . . . and even with the background of that platform, is a missile-for-missile reduction."

In effect, Kinnock appeared to be linking the two main pledges made to him by Chernenko last week. But in the process, he also raised some confusion about whether he was now either backing away slightly from the platform commitment to get rid of Britain's Polaris missile-firing submarines without negotiations or whether he was seeking to make that pledge less controversial by portraying it as having the same effect as a negotiated agreement.

In a statement to the U.S. reporters, Kinnock talked of an "agreement" with the Soviet Union: "Let me make it clear that our only military agreement with the Soviet Union will concern the missile-for-missile reduction in nuclear weapons."