* U.S. President Trump visits Britain for first time

* Most of his visit will take place outside London

* Anti-Trump protests expected in the capital (Updates with comments from U.S. ambassador; adds WASHINGTON dateline)

By Andrew MacAskill and Jeff Mason

LONDON/WASHINGTON, July 6 (Reuters) - President Donald Trump would like a quick trade deal with Britain once Brexit is finished, the U.S. ambassador to Britain said on Friday ahead of a presidential visit to London next week that will keep Trump largely away from planned mass protests.

Trump arrives in Britain on Thursday after a NATO summit in Belgium that could turn contentious over the U.S. leader's insistence that allies pay more for their defense and amid trade disputes between Washington and Europe.

U.S. Ambassador to Britain Woody Johnson painted trade as a potential bright spot in the strong "special relationship" between Britain and the United States, which he said Trump intended to strengthen.

"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," Johnson said, adding that U.S. government agencies were already looking at how to put a deal together.

Less of a bright spot for Trump, whose mother hailed from Scotland, will be the protestors awaiting him in London. More than 50,000 people have signed up to demonstrate against Trump's perceived racism, sexism, and his treatment of migrants. A counter-gathering to welcome him is also planned.

The president will steer clear of the demonstrations.

He will hold talks with Prime Minister Theresa May at her 16th-century manor house, meet Queen Elizabeth at Windsor Castle and attend a black-tie dinner at the home of former World War Two leader Winston Churchill - all outside London.

Johnson said the trip was not planned to avoid the protests. Trump has shown irritation before at protests in the United States.

A spokeswoman for May said the British people were looking forward to his visit.

"We are looking forward to making sure the president has a chance to see and experience the UK beyond London and the south east," she told reporters.

The visit comes at a testing time: May is battling to make a success of Brexit and keep her minority government together, while Trump is challenging Western assumptions about free trade and turning away from global institutions.

Britain regards its close ties with the United States, which it refers to as the special relationship, as a pillar of its foreign and trading policy as it prepares to leave the European Union.

But some Britons see Trump as crude, volatile and opposed to their values on a range of issues. Trump's comments on militant attacks in Britain have sparked anger and he has often exchanged barbs on social media with London mayor Sadiq Khan.

Protesters are planning to fly a blimp over parliament portraying Trump as an orange, snarling baby during his visit after Khan approved a request for its use.

On his arrival on Thursday afternoon, the president will travel to Blenheim Palace, the 18th-century mansion where Churchill was born and spent most of his childhood, eight miles (12 km) north of Oxford, according to May's office.

In the evening, May will host a black-tie dinner for Trump at the stately home that will be attended by about 100 business leaders from industries including finance, pharmaceuticals, defence and technology.

For the only time during his visit, Trump will then travel into London when he will stay overnight at the home of the U.S. ambassador in the centre of the city.

On Friday, Trump and May will visit an undisclosed location to witness a display by British soldiers.

Trump will travel with May to Chequers, the prime minister's official country residence. He will then go to meet the Queen at Windsor Castle, the family home of British kings and queens for almost 1,000 years.

Afterwards, the president will travel to Scotland, where he owns two golf courses. Trump's wife, Melania, will have a separate itinerary and will be hosted by May's husband Philip. (Additional reporting by William James in London Editing by Matthew Mpoke Bigg)

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