Queen Elizabeth has seen a lot in her 92 years, from Nazi bombers to nasty bummers when her children’s marriages dissolved in scandal, plus every US president since Dwight Eisenhower, who paid a visit at the start of her marathon reign.

But chances are she has never before encountered any caller quite like Donald Trump, who stopped by Windsor Castle on Friday to pay his colonial respects with First Lady Melania in tow.

While what went on at their private encounter over tea and cucumber sandwiches will forever remain behind the portrait-lined walls of the royal residence, the slightest touch of imagination is enough to make the mind boggle.

On one side of the table, the epitome of stiff-upper lip Britishness.

On the other, a petulant, arrogant, orange-haired braggart fresh from stunning Continental leaders with demands and implied threats that they had better contribute more to the NATO defence budget or the US might just pack up its tanks, planes and military bases and go home.

Queen Elizabeth II with US President Donald Trump and First Lady Melania Trump stand for the national anthem at Windsor Castle. Photo: Getty

So, did President Trump, who boasts of not playing by the same rules as other political leaders, stick to court etiquette? Did he slurp from her Majesty’s bone china tea cups? Did he sustain that oversize waistline with more than his share of petite fours?

Or did he tell the Queen that her favourite palace was “a nice  joint” but one that could be better if she followed his personal style cues and added the equivalent of Kalgoolie’s annual gold  production to walls, floors and bathrooms to brighten the place up?

Knowing Mr Trump, he may well have.

For Mr Trump, who was simultaneously being mocked by human protesters and a six-metre helium balloon depicting him as a bawling, selfish, orange-tinted baby in an oversize diaper, biting his tongue must have required a supreme effort.

The helium-filled Trump blimp takes to the skies of London above a crowd of chanting, jeering protesters. Photo: CrowdSpark/See LI

According to the UK Telegraph, the Trumps spent 57 minutes inside Windsor Castle with the Queen before being driven away in a motorcade of Range Rovers.

Neither spoke after their meeting but the trio posed for a photo in which the Queen of the United Kingdom managed a vacant smile.

First picture of @realDonaldTrump with the Queen inside Windsor Castle #TrumpVisitsUK https://t.co/wCmct6S820 pic.twitter.com/BUB4uqgZya

— ITV News (@itvnews) July 13, 2018

Outside Windsor, a legion of demonstrators chanted and waved placards, including “Beware Ma’am He’s a Sociopath” and “Off With His Head!”

Police, too, were busy, with massive cordons and security checks throughout London and an equally large presence at Windsor, where Mr Trump arrived by helicopter shortly after 3pm on a sunny English afternoon.

“I especially feel for the poor old Queen having to have him to tea,” said protester Jenni Collins, 36.

Angry signs and furious voices fill Trafalgar Square to overflowing. Photo: EPA/Andy Rain

As for the Queen she was, as always, the essence of regal discretion.

The monarch smiled while receiving Mr Trump and First Lady Melania Trump in the quadrangle of Windsor Castle before inspecting a guard of honour with the US leader.

It was a significant downgrade from the last visit by a sitting US president when Barack and Michelle Obama were treated to a state dinner at Buckingham Palace in 2011.

The inspection of the guards was also not without incident when Mr Trump abruptly stopped in front of the Queen, forcing her to walk around him.

.@POTUS and @FLOTUS arrive at Windsor Castle to meet with Queen Elizabeth II. https://t.co/zPd2SMttxw

— Fox News (@FoxNews) July 13, 2018

Still, no matter how bizarre and incongruous the tea for three might have been, connoisseurs of foot-in-mouth moments will always regret that the man who might have made Mr Trump feel right at home skipped the Earl Grey and stayed home.

We’re talking of course about 97-year-old Prince Philip, who recently retired from public life after a lifetime of saying the most outrageous things to the most inappropriate people – once telling an African leader in ceremonial white robes, to cite but one example, that he looked “ready for bed.”

As Donald Trump makes his way to Windsor Castle, The Queen gives one member of staff a 'special assignment'. pic.twitter.com/YDKV0V7QN5

— Have I Got News For You (@haveigotnews) July 13, 2018

Given his remarks about Indian electricians, Chinese facial features and Aborigines waving spears, the Duke of Edinburgh and the US President might well have got on like a palace on fire.

By the time they got around to joking about Mexicans, Muslims and Euroweenies, Mr Trump would felt almost like a member of the family.

Mr Trump will now spend the next two days golfing at his resort in Scotland.