Britain runs either an $89 billion trade surplus or a $14.5 billion deficit with the United States, depending on whether one cites British or American statistics. The difference rests in part on how the two sides treat offshore financial centers such as Jersey and Guernsey, which are crown dependencies.

“He was working hard, I’ll tell you that,” Mr. Trump said, after Mr. Starmer lobbied him against tariffs at a White House meeting late last month. “He earned whatever the hell they pay him over there.”

Mr. Starmer has also been pushing Mr. Trump to provide American security guarantees to Ukraine as part of a peace negotiation with Russia. The two leaders have spoken regularly by phone since their meeting, as Mr. Starmer has tried to help heal Mr. Trump’s rift with President Volodymyr Zelensky of Ukraine.

But Britain’s decision not to retaliate to the tariffs could complicate Mr. Starmer’s other big priority: to draw his country closer to the European Union after Brexit. The crisis over Ukraine has given the prime minister a chance to collaborate with the European Union on defense and security, and he clearly hopes it could lead to closer trade and economic links.

The divergent responses to the tariffs are a reminder that, in some respects, Britain still faces a choice between the United States and Europe.