The two leaders will hold talks the following day at Chequers, the 16th-century manor house which is the prime minister's official country residence.
After a North Atlantic Treaty Organisation summit where he provoked a crisis session to force allies to raise their defense spending, Trump landed in Britain having described the closest US ally in Europe as being in turmoil over Brexit.
Trump had words of praise, however, for former British Foreign Minister Johnson who resigned earlier this week over the soft Brexit strategy, saying Johnson would "make a great prime minister".
The plan has infuriated fervent Brexit supporters, who think sticking close to the bloc would limit Britain's ability to strike new trade deals around the world.More news: First Israeli spacecraft to land on moon next year
"If they do a deal like that", Trump said, "we would be dealing with the European Union instead of dealing with the United Kingdom", adding that approach would "probably kill" any future trade deals with the U.S. "The people voted to break it up so I imagine that is what they would do, but maybe they're taking a different route". She should negotiate the best way she knows how.
The pound has fallen more than 0.5% against the dollar on Friday morning.
"I guess when they put out blimps to make me feel unwelcome, no reason for me to go to London", he said.
Tens of thousands were expected to take part in a demonstration later in the day in London, as well as in other British cities.
Trump will later Friday take tea with Queen Elizabeth II at Windsor Castle, before spending the weekend in Scotland on a private visit that will likely take in 18 holes at one of the property magnate's golf courses, where more protesters await him.
The interview was the latest breach of diplomatic protocol by Trump, whose predecessors tended to avoid criticizing their foreign hosts.More news: How much do we really spend?
The Trumps were greeted in the courtyard of Blenheim Palace in Oxfordshire by UK Prime Minister Theresa May and husband Philip for the dinner in the President's honour. I think he has done a very bad job on terrorism.
The controversial 45th President of the United States will visit the UK for the first time in his position on Thursday, July 12.
"As he said in his interview with the Sun she "is a very good person" and he "never said anything bad about her", she said. It is not considered appropriate to call the Queen by her first name - a gaffe that Nelson Mandela made in 2014 - and it is never okay for people to use her nickname Lilibet, which is reserved exclusively for very close family members.
10K people backed the Trump Baby petition, which asked London's mayor Sadiq Khan to give permission to let the blimp soar in the sky. "I think you are losing your culture". The first lady will remain in London on Friday when protests are set to take place. "But when they make you feel unwelcome, why would I stay there?"
Trump, 72, was dressed in a smart, black tuxedo while Melania, 48, chose a glamorous, off-the-shoulder lemon gown with ruched detailing across the bodice, a full skirt and a cape overlay.More news: Now-Hurricane Chris Poses Possible Threat to Nova Scotia and Newfoundland-Labrador
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