The four-day trip, which will include talks with Prime Minister Theresa May, tea with Queen Elizabeth II and a private weekend in Scotland, is set to be greeted by a leftist-organised mass protest in London tomorrow.
"When we leave the European Union we will begin to chart a new course for Britain in the world and our global alliances will be stronger than ever," May said ahead of Trump's visit.
"There is no stronger alliance than that of our special relationship with the US and there will be no alliance more important in the years ahead," the British leader said.
Britain "is in somewhat turmoil", Trump said before departing Washington, remarking that dealing with Putin might surprisingly be the easiest part of the European trip.
"The UK certainly has a lot of things going on," he said, referring to the resignations of Britain's Brexit and foreign ministers over the government's plan for ties with the European Union after it leaves the bloc in March.
Ambassador Johnson sought to explain Trump's comments.
"There's always turmoil in every country but no, no, I think the UK is proceeding as it always does," he told BBC radio.
"We're extremely confident in the ability of the UK to plough through this issue with Brexit and move on," he said.
On the planned protests, the ambassador said that Trump appreciated free speech and dismissed as "irrelevant" a giant balloon of Trump depicted as a crying baby in nappies which will fly next to the British parliament tomorrow.
The crowdfunded initiative has been dubbed "Trump Baby".
The US president's brash style and hardline "America First" policies have caused consternation across Britain's political spectrum and society.
His criticism of Britain's anti-terror policies after a series of attacks in 2017 also did not go down well.
Opposition lawmakers, backed by an online petition signed by nearly 1.9 million people, called on May to cancel the state visit offered when she met Trump in Washington after his inauguration in January last year.
May will seek to put the diplomatic tensions behind her when she hosts Trump today for a black-tie dinner with business leaders at Blenheim Palace, the birthplace of World War II prime minister Winston Churchill.
He is due to stay overnight with Johnson at the ambassador's Winfield House residence in London's Regent's Park.
Tomorrow, May and Trump will visit a defence site then travel to the prime minister's Chequers country residence for talks followed by a press conference.
The pair will discuss relations with Russia, Brexit and trade ties, Downing Street officials said, after Trump recently slapped tariffs on EU steel and aluminium imports.
There will be small-scale protests all along Trump's itinerary and a mass demonstration called "Together Against Trump" by left-wing campaign groups in London tomorrow.
"We are going to organise a massive national demonstration against his politics of sexism, racism, war, hate and climate denial," organisers said.
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