A speedy post-Brexit trade deal between Britain and the United States is a major priority for President Donald Trump, the US ambassador to Britain said on Friday ahead of the president's visit to London next week.

Trump is steering clear of the mass protests in central London that will mark his first visit to Britain as president on July 12 and 13. Instead he will meet dignitaries outside the capital at country residences and a castle.

"He would love to do a bilateral trade deal and he is really ready to step up on that the minute we get the go ahead to do it. He will get it done fast because I know it is a major priority for him," US Ambassador Woody Johnson said in a conference call with reporters.

Asked about the president's thoughts on Brexit, the process of leaving the European Union that is putting British Prime Minister Theresa May's leadership under pressure, Johnson said the United States did not favour any particular form of EU exit.

"We're not advocating hard or soft - whether it's hard or soft (Brexit) we know that Great Britain is going to do very very well," he said.

"Britain has a once-in-a-lifetime opportunity to change direction and I am very confident whichever way this goes that Britain will make a success of Brexit and whatever else it does in the future because the UK is loaded with talent."