So much for sweet summer memories.

President Trump made two trips to Britain in the last four months, once to visit his Scottish golf resorts in June and a second time last week, when he was feted by King Charles III at Windsor Castle. And yet, on Tuesday, Mr. Trump told European leaders at the United Nations, “Your countries are going to hell.”

Mr. Trump didn’t single out Britain, but he didn’t have to. His reference was unmistakable. Mr. Trump said Europe needed to “end the failed experiment of open borders,” echoing a public warning he delivered to Prime Minister Keir Starmer about immigration at the end of his state visit.

The president did not stop there. He also reopened a long-simmering feud with London’s mayor, Sadiq Khan, who he suggested, falsely, was seeking to impose Shariah law.

“I look at London, where you have a terrible mayor, terrible, terrible mayor, and it’s been changed, it’s been so changed,” he said. “Now they want to go to Shariah law. But you are in a different country, you can’t do that.”

It was not clear how Mr. Trump formed his impression of a repressive, incompetently run London: He spent barely 12 hours in the city, overnighting at the baronial residence of the American ambassador in Regent’s Park, before flying by helicopter to Windsor Castle and then to Chequers, Mr. Starmer’s country retreat.

The carefully curated itinerary saved Mr. Trump any contact with protesters, wielding banners that said, “Dump Trump.” When an activist group projected images on the walls of Windsor of Mr. Trump and Jeffrey Epstein, the financier and sex offender, with whom he once socialized in Palm Beach, Fla., the police quickly arrested four men.

Spare a thought for Mr. Trump’s hosts, who spent months planning a visit to remember for their mercurial guest: Horse-drawn carriages, columns of Grenadier guards, a white-tie banquet, a flyover by Red Arrow aerobatic planes, parachutists trailing British and American flags.

To read the British papers was to feel offended on behalf of hardworking hosts who had just gotten a negative review on Yelp or TripAdvisor.

“Trump: West going to hell over migration,” said The Daily Telegraph, which noted his attack on the mayor. “Your countries are going to hell, Trump tells European leaders,” said the banner in The Guardian. The tabloid Daily Mirror went with a photo of the president and the one-word headline, “Deranged.”

For Mr. Starmer, who has made it his mission to get along with Mr. Trump, no matter what the cost to national dignity, the harsh comments will resurface questions about whether it’s time for a “Love, Actually” moment.

In Richard Curtis’s 2003 film, a British prime minister, played by a polite Hugh Grant, tells off a swaggering, uncouth American president, played by Billy Bob Thornton, for taking advantage of an ally. “A friend who bullies us is no longer a friend,” Mr. Grant declares, as his aides tear up in patriotic pride.

Nobody expects that from Mr. Starmer, a reserved lawyer who has dismissed calls to confront Mr. Trump as performative and counterproductive. Last March, after the president berated President Volodymyr Zelensky of Ukraine in the Oval Office, Mr. Starmer said, “we were under pressure to come out very critically with, you know, flowery adjectives to describe how others felt.”

“I took the view that it was better to pick up the phone and talk to both sides to try and get them back on the same page,” he said in an interview, adding, “I like and respect him. I understand what he’s trying to achieve.”

Mr. Khan, who is London’s first Muslim mayor, has not quite turned the other cheek. After initially declining comment, he called Mr. Trump “racist” and “Islamophobic” in an interview on Wednesday morning, and said people would wonder why he “appears to be living rent free inside Donald Trump’s head.” And he pointed to official data that shows a record number of Americans are settling in Britain.

Mr. Trump is not the only American who has run down Britain, only to savor its pleasures. Vice President JD Vance brought his family to the country in August for a vacation in the Cotswolds, a scenic area west of London, and a weekend at Chevening, the country residence of the foreign secretary.

David Lammy, who was then serving in the job, took Mr. Vance carp fishing, and laughed obligingly when his guest said he caught more fish than Mr. Lammy. This is the same JD Vance who had suggested a few months earlier that Britain was “some random country that hasn’t fought a war in 30 or 40 years” (He later insisted he was not talking about Britain or France).

William Hanson, an etiquette coach who recently published a book, “Just Good Manners,” said his country could absorb the backhanded treatment by the United States, even if it strikes many Britons as bad form. Rather than retaliating, he suggested that Britain pursue a “slight subtle withdrawal” from Mr. Trump’s America.

Of course, Mr. Hanson added, “The Trump administration may not respond to subtlety.”