Mr. Price, who is now managing director of Rock Creek Global Advisors, an economic advisory firm, said, “This negotiation seems more driven by politics at the top than by U.S. or U.K. commercial interests.”

For Mr. Trump, a trade deal is a reward for Mr. Johnson’s championing of Brexit, which undermines the European Union, a supranational project he deeply distrusts. For Mr. Johnson, a deal is Exhibit A of the new path that Britain can forge, having thrown off the shackles of Brussels.

That is not to say there are no concrete economic benefits. The British government said Scotland could export more Scotch and smoked salmon, northern England more cars, and the Welsh more lamb to the United States.

Both countries are competitive in industries like finance and digital services; executives in those industries say they could benefit writing rules looser than those of the European Union. But Britain must still decide whether to tax American technology firms like Google, Facebook and Amazon.

The Trump administration will push to reduce barriers to its farm products and press the National Health Service to pay more for American drugs — both of which raise vexing political issues in Britain.

In its blueprint, the government said, “the NHS is not, and never will be, for sale to the private sector, whether overseas or domestic.” But the Labour Party and other critics warn that Mr. Johnson will accept chlorinated American chicken and allow American companies to buy chunks of the revered health service.

British officials have hinted at more flexibility on American food. Mr. Lowe, the trade expert, noted that the blueprint commits only to preserving Britain’s high standards in food safety — not the existing regulations that keep out American poultry.