Image copyright Reuters Image caption Mr Trump's trip to Scotland comes at a delicate time for the UK and the EU

Remember the scene in the Godfather where Michael Corleone says "it's not personal, it's strictly business?" Well you could rework that quote for Donald Trump, except in his case it would be "it's not politics, it's business".

How else do you explain his arrival at the wonderful links course on Scotland's west coast?

To be clear, there is nothing unusual about a US presidential nominee doing a highly-condensed, gap year-style inter-rail tour around the capitals of Europe, glad-handing with the ruling elite, showing off your grasp of knotty global issues and buttering up the leaders you will be spending the next four years with at interminable G7s, G20s, Nato councils, UNGAs and nuclear security summits. It's what you do.

President Obama rocking the Brandenburg Gate, Mitt Romney offending his hosts at Downing Street for dissing the 2012 London Olympics - and all sorts of others pass through. New Jersey Governor Chris Christie came through London and went to watch Arsenal play (always had my doubts about him after that), Wisconsin Governor Scott Walker came and made news by talking about creationism. The path is well-trodden.

Image copyright AP Image caption Senator Barack Obama was cheered by thousands in Berlin in 2008

But Donald Trump's trip is - well - so Donald Trump. It is unorthodox, flies in the face of convention, and a presumptive nominee could not be visiting the UK at a more delicate time politically.

To the purpose of the visit - he is going to cut the ribbon on a refurbished Turnberry golf course, another gleaming part of the Trump business empire. The visit comes as his campaign has been going through its rockiest patch. Assailed by his detractors, seemingly short of cash, his campaign manager's been fired, his poll numbers heading south as he heads to Britain's north. I am sure many of his advisors were wishing he was staying at home.

He will fly out of New York with Britain still a member of the EU, and land into Prestwick with the country having potentially voted to withdraw. But none of this is going to form part of his trip. As far as we know he has neither sought nor been offered meetings with senior ministers - this is strictly business.

Image copyright Reuters Image caption Donald Trump lands in Scotland amid the upheaval of the EU referendum

And that is what is so extraordinary and so exceptional about Mr Trump. Can you name me any other presidential hopeful who has done anything like this? Even close? No. Didn't think so.

That said I am sure it is a huge relief to the government protocol officers. I'm sure Trump's people would have been given pretty short shrift from the Downing Street diary secretaries if they'd asked for a slot on Friday when who knows what might be unfolding?

And surely there is something laudable about being able to keep politics and business separate. Except that it is very difficult to do.

Look at his little local difficulty over Trump University where he is facing allegations of defrauding students. That hasn't worked out so well.

He was scathing in his criticism of federal Judge Gonzalo P Curiel on a number of occasions for being unable to deliver a fair verdict because of his "Mexican" heritage. Mr Curiel was born in Indiana - quite some distance from the Rio Grande. His remarks had Republican donors and grandees running for cover with a chorus of criticism that Mr Trump's comments were racist.

Image copyright European Photopress Agency Image caption House Speaker Paul Ryan said Trump's comments were the textbook definition of racism

Or take the charge of hypocrisy levelled at Mr Trump over a lot of the branded merchandise that he flogs at his golf courses not being made in the USA when he says he wants to penalise companies for outsourcing. Again -sometimes politics and business don't mix very easily.

And maybe he will nip back before November to speak to David Cameron where there is a little bit of fence-mending to be done. After Mr Trump said he wanted to ban all Muslims, Mr Cameron said those comments were stupid, wrong and divisive. Though noticeably the language has warmed since then.

Donald Trump is bound to want to come knocking on the famous black door of Downing Street very soon - and to quote the Godfather again, that might be an offer Mr Cameron can't refuse.

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