♦ UK, US agree to engage Russia with strength and unity: May

♦ Trump looks forward to finalising a post-Brexit  trade deal with Britain

US President Donald Trump said yesterday ties between Britain and the US were "very very strong" after launching an extraordinary attack on Prime Minister Theresa May's Brexit strategy during a visit already marked by protests.

Sitting alongside his host at the start of talks at May's country retreat of Chequers, Trump told reporters: "The relationship is very, very strong. We really have a very good relationship."

Activists inflate a giant balloon depicting US President Donald Trump as an orange baby during a demonstration against Trump's visit to the UK in Parliament Square in London yesterday. Photo: AFP, Reuters

Their meeting came just hours after an interview was published in which Trump said May's plans for close future ties with the European Union would "probably kill" her hopes for a US trade deal.

He suggested this was not what voters wanted, said he had advised May to take another path and added that Boris Johnson, who quit as foreign minister over the plan, would make "a great prime minister".

Trump's comments saw the value of the pound plunge on currency markets.

Trump said that at a gala dinner at Blenheim Palace the night before he and May spoke for almost 90 minutes and had "probably never developed a better relationship".

The prime minister smiled and said there was "a lot to discuss", including "the special relationship, which is great, between the UK and US".

She said they would discuss "the real opportunities" on a US trade deal, as well as foreign, defence and security issues "where we work really closely together".

A young demonstrator holds a banner objecting to the visit, in Belfast, Northern Ireland. Photo: AFP, Reuters

After the meeting, Trump said he was "OK" with May's Brexit strategy and was hopeful the two countries could strike a trade deal, a day after excoriating her policy.

"Whatever you do, it's OK with me," he said after talks with May, adding: "The only thing I ask of Theresa is to make sure we can trade."

May said Britain and the United States have agreed that Russia should be engaged with "strength and unity".

Trumps meet Britain's Queen Elizabeth II (C) in the Grand Corridor at Windsor Castle in Windsor, west of London. Photo: AFP, Reuters

"We agreed that it is important to engage Russia from a position of strength and unity," May said, as Trump prepares for a summit with Russian President Vladimir Putin on Monday.

Trump's interview with The Sun came before he flew into Britain on Thursday from Brussels, where he subjected Nato allies to a roasting over their defence spending.

US First Lady Melania Trump (C) high-fives with a British military veteran known as a "Chelsea Pensioner" during a game of bowls at the Royal Chelsea Hospital in London. Photo: AFP, Reuters

Junior foreign minister Alan Duncan sought to brush off the remarks, telling the BBC: "Donald Trump is a controversialist, that's his style... I don't think we see it as rude."

In London, demonstrators began gathering for a mass protest against Trump, beginning with the flying of a huge balloon of the president depicted as a crying baby outside the Houses of Parliament.

"I think it's hilarious. It's an accurate representation of his politics which are so immature. He never enters into adult discussion," said Paul Fonseca, 23.