The image was unfortunate: Prime Minister Keir Starmer, crouching down to scoop up papers spilled by President Trump as he showed off a finalized trade agreement between the United States and Britain. Yet the inevitable comparisons of the British leader to an eager courtier belied a surprisingly successful outcome for Mr. Starmer at the Group of 7 meeting in Alberta, Canada.

In addition to the trade deal, Mr. Starmer won Mr. Trump’s assurance that the United States would not mothball a strategically important submarine alliance with Britain and Australia. Worries about the future of the alliance, known as AUKUS, had flared in London last week after the Pentagon began a review of it.

At a leaders’ gathering where Prime Minister Mark Carney of Canada cut short a photo opportunity with Mr. Trump and the president said on social media that a “publicity seeking” President Emmanuel Macron of France “always gets it wrong,” Mr. Starmer emerged, improbably, as Mr. Trump’s best friend.

As often with Mr. Trump, friendship can feel rather casual. After telling reporters that he and Mr. Starmer had signed an executive order implementing the British-American deal, which was first announced in May, Mr. Trump said, “So we have our trade agreement with the European Union.”

Mr. Starmer blinked and kept his silence.

The executive order left several details unresolved: While it lowered American tariffs on British cars to 10 percent from 25 percent — a critical goal for Mr. Starmer — Mr. Trump stopped short of guaranteeing zero tariffs for British steel exports. And digital services remain outside the scope of the agreement, as do specific tariffs on pharmaceuticals.