British Prime Minister Theresa May headed to the United States on Thursday where she will be the first foreign leader to meet President Donald Trump, with post-Brexit trade top of the agenda.

There are high hopes in Westminster for the visit as she has promised a thriving economy after Britain leaves the European Union and the meeting will be closely watched in Washington for clues on Trump's foreign policy.

While Trump has been an avid supporter of Brexit, which he believes mirrors his own anti-establishment rise to the White House, his criticism of mainstream politics and calls for a protectionist trade regime could prove problematic for May.

The prime minister will meet Trump on Friday in Washington, where the duo will discuss a bilateral trade deal, after stopping off in Philadelphia on Thursday to meet Republican Party leaders.

The British premier will also take gifts -- a hamper of products from the prime minister's official country residence, Chequers, for First Lady Melania Trump and a Scottish "Quaich" cup for the president.

Downing Street said the double-handled cup has its origins in "Highland chivalry", from a time when "clan chiefs prized them as a token of hospitality".

Trump's mother came from Scotland and the sprawling property empire he built up includes two Scottish golf courses.

May's spokeswoman said the visit is intended to "establish a productive, effective relationship" with the Trump administration.

"As the prime minister has said, the way we will approach this trade union relationship with the UK in the future is what is in the interest of the UK," the spokeswoman said.

Both have expressed their enthusiasm for a quick deal, but under EU rules Britain must wait until it has formally left the bloc -- after an expected negotiating period of two years -- before it signs agreements with other states.

- 'Not afraid to speak frankly' -

Beyond trade, the two leaders will discuss terrorism, the ongoing war in Syria, and the NATO military alliance which Trump deemed "obsolete" ahead of taking office last Friday.

His criticism has caused alarm across the Atlantic and earlier this week May spoke to NATO chief Jens Stoltenberg, expressing her support for the alliance and promising to take the message to Washington.

Although Britain is keen to maintain the "special relationship" with the US, particularly in light of economic uncertainty post-Brexit, wariness of Trump remains in the UK where he has proven a controversial figure.

Last January parliament debated banning the billionaire from Britain, sparked by a public petition which garnered nearly 600,000 signatures, following his proposal to drastically restrict US entry to people from Muslim countries if he became president.

British lawmakers discussed May's upcoming visit on Wednesday, with opposition Labour leader Jeremy Corbyn quizzing her on the trade deal and allegations against Trump of misogyny.

"I'm not afraid to speak frankly to a president of the United States," she replied.

Asked about whether she supported Trump's comments in favour of torture, she said: "I can assure you that we have a very clear position on torture. We do not sanction torture, we do not get involved with that and that will continue to be our position".