As the staff at Blenheim Palace cleared away the plates last night and guests discussed the possibilities for British and American trade, the front page of The Sun was landing electronically in London. The headline was a corker.

“May has wrecked Brexit… US trade deal is off!”

In an interview, Trump told the paper that he had told May how to handle Brexit properly but she failed to follow his advice. The British proposed compromise outlined in the White Paper (I’m sure the President has read it all, or maybe not) means that a trade deal with the US is off. He’ll just have to deal with the EU instead.

“I would have done it much differently. I actually told Theresa May how to do it but she didn’t agree, she didn’t listen to me. She wanted to go a different route. I would actually say that she probably went the opposite way. And that is fine. She should negotiate the best way she knows how. But it is too bad what is going on.”

The government’s Brexit proposal – assailed from all sides – is death to a deal with the US, a deal Trump has been keen to do because he likes the UK and sees Brexit as synonymous with his own anti-establishment, disruptive approach to governing.

“If they do a deal like that, we would be dealing with the European Union instead of dealing with the UK, so it will probably kill the deal.”

What a scoop from Tom Newton-Dunn, the paper’s political editor.

It’s a great story – a market mover. But just as Barack Obama’s arrogant and misfired intervention during the referendum campaign in 2016 on behalf of Remain was disgraceful, Trump’s intervention in domestic British affairs on our soil is an appalling insult to his hosts, the British government, and by extension disrespectful to the British more broadly.

Not only is it the height of oafish bad manners, accepting an invitation to dinner and trashing your hosts hours before in a newspaper interview. It represents an intervention in the sovereign affairs of a country that has been nothing but friendly and generally supportive towards the United States for more than a century.

As it happens, May should have pursued a different and more sophisticated strategy from the start, including an open and generous unilateral declaration on EU nationals and full preparations for a fallback to “no deal” in case. But it is not the proper role of a foreign head of state to come to this country and lecture the Prime Minister in public.

Perhaps inevitably – is it the heat? – some of the hardest Tory Brexiteers have hailed Trump’s intervention. Their thinking – such as it is – runs as follows.  It was appalling when Obama intervened in the referendum, but this is okay, because… because… because… because of the wonderful things he does.

Increasingly, the most hardline MPs seem to exist in a Wizard of Oz fantasy world in which at the end of the Yellow Brick Road there is a magical majority in parliament and the country for a deliberate “no deal” Brexit, and no attempt to get a deal or even a transition, and then a magical trade deal with the US next April and perhaps India – lowering food safety standards and opening up masses more migration. Honestly, think about it. Toto, I’ve a feeling we’re not in Kansas anymore.

Some of the hardest Brexiteers seem to be driving themselves completely round the twist. Backing Trump over a UK government. Threatening to kill the White Paper in the vague hope that, in the manner of revolutionaries down the ages, it leads to an eruption that could even let in Corbyn if it goes really badly wrong.

I’ll write more on this in my weekly newsletter for Reaction subscribers later today. (We have a half-price summer sale on, only ÂŁ24 for the year.)