“I fear that this has become a bad relationship — a relationship based on the president taking exactly what he wants and casually ignoring all those things that really matter to, um, Britain,” he says in the movie.

“We may be a small country, but we’re a great one too,” he adds, to a soundtrack of soaring strings. “A friend who bullies us is no longer a friend. And since bullies only respond to strength, from now onward I should be prepared to be much stronger. And the president should be prepared for that.”

Mrs. May, who hosted Mr. Trump at a dinner on Thursday and was meeting with him at the country estate Chequers on Friday, showed no sign of planning a confrontation. But at least one prominent supporter of the opposition leader, Jeremy Corbyn, said that he would have done so: