Prime Minister Theresa May is to hold talks with US President Donald Trump in the Oval Office later.
The pair will spend about an hour together in the first visit by a foreign leader to the White House since Mr Trump became president.
Earlier, Mrs May told US Republicans the UK and US could not return to “failed” military interventions “to remake the world in our own image”.
But she said they should not “stand idly by when the threat is real”.
- Laura Kuenssberg: Do opposites attract?
Post-Brexit trade opportunities, security and intelligence co-operation and the future of Nato are likely to feature prominently in Mrs May’s talks with Mr Trump.
Although the UK cannot begin to negotiate trade deals with the US or other countries until it leaves the EU, Mr Trump has said he wants a “quick” deal after that, and the two leaders are expected to discuss future co-operation.
Chancellor Philip Hammond stressed the UK would abide by EU rules on trade talks when he arrived in Brussels for talks earlier.
“Of course we want to strengthen our trade ties with the very many partners we have around the world but we are very mindful of our obligations under the treaty and we will follow them precisely,” he said.
In her speech to US Republicans on Thursday, Mrs May said she wanted to “renew the special relationship” between the UK and America.
She said the two countries “must always stand up for our friends and allies in democratic countries that find themselves in tough neighbourhoods too”, to applause from her audience.
But Tony Blair’s former chief of staff Jonathan Powell said Mrs May “seemed to contradict herself” by warning against foreign intervention, but then saying the US and UK must uphold their values around the world.
He said “it would be a mistake to encourage Donald Trump to be isolationist” when he was already talking about “America first”.
But Defence Secretary Sir Michael Fallon told BBC Radio 4’s Today programme: “This is the relationship that matters above all to our country and to its future prosperity.”
On military interventions, he said: “What she’s made clear, in future, is that where we are going to commit British troops, there has to be a very real threat to our country.
“It has to be in the British national interest that we intervene in these countries… We should not simply embark on foreign adventures for the sake of it.”
By BBC political editor Laura Kuenssberg
Last night Theresa May signalled a turn away from American and British foreign policy of recent years – military interventions into countries like Afghanistan and Iraq.
But also, she urged President Trump not to turn America’s back on the world and not to allow the rise of Asian nations to “eclipse the West”.
For the president who has vowed again and again to put “America First”, it was an effort to persuade – if not outright criticism. Yet she also heaped praise on Donald Trump’s victory.
Well aware of the dangers, Theresa May is nothing if not ambitious for her relationship with her new political friend.
US Congressman Kevin Cramer, who was in the audience for Mrs May’s speech told the BBC: “As I was watching her, and listening to her, I thought, ‘Is this Donald Trump’s long lost sister?’
“There were real similarities, while at the same time, clearly a different take on certain global issues.”
The speech followed comments by UK Foreign Secretary Boris Johnson to a House of Lords committee on Thursday that Bashar al-Assad should be allowed to run for election to remain in power in Syria – a reversal of UK foreign policy.
Mr Johnson told the committee that Britain might have to “think afresh” about how to handle the Syrian crisis and had failed to enforce its “mantra” that “Assad must go”.
He said: “I see downsides and I see risks in us going in, doing a complete flip flop, supporting the Russians, Assad.
“But I must also be realistic about the way the landscape has changed and it may be that we will have to think afresh about how to handle this.”
On Thursday, the prime minister also spoke in support of Nato – which Mr Trump has called “obsolete” – and the Iran nuclear deal, which Mr Trump threatened to scrap during his campaign.
And she said care must be taken to distinguish between “extreme and hateful ideology” of Islamist extremism and the “peaceful religion of Islam and the hundreds of millions of its adherents”.
On dealing with Russia’s President Putin, Mrs May said: “My advice is to engage but beware.”
The visit comes amid controversy over comments by President Trump about waterboarding.
He told ABC News on Wednesday that after he asked intelligence officials if torture “works” he was told, “‘Yes, absolutely.'”
Mrs May has said the UK condemns torture and told journalists: “My view on that won’t change – whether I am talking to you or talking to the president.”