London — Some 15 million Britons spent Saturday evening watching the US television film ''The Day After,'' which shows the devastating effect of a nuclear attack on a small town in Kansas.
That figure underlines the importance the British attach to the nuclear debate, particularly with US cruise missiles now deployed on British soil. But it was still only one-third of the number of British viewers who watched the televised wedding of Prince Charles to Lady Diana Spencer in 1981.
The switchboard of Yorkshire Television, the commercial TV station that broadcast the ABC film, was jammed with calls after the show. A station spokesman said they were fairly evenly divided between those who felt it should not have been shown and those who found it a worthwhile exercise.
The question now being asked is: Did the film change any minds?
The British defense secretary, Michael Heseltine, who had been given a private showing of the film a week ago, clearly felt it might. He said the danger was that the film ''might whip up public emotions and give the Campaign for Nuclear Disarmament a degree of support that all their arguments have failed to achieve.''
Although he urged people to watch ''The Day After,'' he described it as politically biased and asked to be allowed to comment on television - the first time a British politician has demanded the right to speak on the contents of a fiction film. This set off a week-long battle with the authority that governs commercial broadcasting in Britain, which said his request unreasonable.
It was also the first sign that the showing of the movie was to be the center of a high-level political debate. This was confirmed when the Campaign for Nuclear Disarmament (CND) was not allowed to advertise during the commercial breaks in the film, as it had planned to do.
Mr. Heseltine got his way and, in a TV interview after the film, reminded viewers that the West's existing defense policies have kept the peace in Western Europe ''for an unprecedented period in contemporary history.''
He declined to take part in the studio discussion that followed the movie's screening because the chairperson of the CND was also to appear. Mr. Heseltine has made it a policy not to share a platform with any CND figures.
In the end, the panel discussion merely gave British politicians and the CND an opportunity to restate their views on British defense policy. Each side insisted the film had proved its own case.
The only people who can be expected to draw any real satisfaction from Saturday night's events are British video cassette distributors, who say ''The Day After'' and the controversy that surrounds it will be good for business. About 10,000 video copies of the film were to go on sale Sunday in shops throughout the country.