For Prime Minister Keir Starmer of Britain, the news of an imminent trade agreement with the United States is a much-needed political victory, as well as vindication of his strategy of assiduously cultivating President Trump.

The details of the agreement were not yet clear and could prove contentious with farmers and other interest groups in Britain. Mr. Trump’s announcement on social media seemed to catch even senior British officials off guard, after weeks of negotiations characterized by two-steps-forward, one-step-back progress.

But that Britain is the first country in the world to conclude a deal with the United States after Mr. Trump imposed an across-the-board tariff of 10 percent on dozens of American trading partners seemed to affirm the “special relationship” often invoked by British and American diplomats.

In his post on Wednesday evening, Mr. Trump heralded a “MAJOR TRADE DEAL WITH REPRESENTATIVES OF A BIG, AND HIGHLY RESPECTED, COUNTRY.” He said the deal would be announced on Thursday morning in Washington.

Britain, even under a center-left Labour government, has escaped the harsh criticism that Mr. Trump has trained on the European Union and China, both of which he accuses of “ripping off” the United States in their trade relationships.

That partly reflects economics: Britain and the United States have reasonably balanced trade flows. 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.