The president is unlikely to see the balloon as he made his journey by helicopter from the US ambassador's residence in Regent's Park to watch a military display at Sandhurst, the famous academy for the training of military officers.
In an interview with British newspaper The Sun in which Trump also attacked Prime Minister Theresa May over her strategy for exiting the European Union, the president said that the queen had "never made a mistake" during her long reign.
Labour's Stephen Doughty said: "An image of Donald Trump sitting in Churchill's seat will stick in the throats of many Brits who would consider Trump not only to be one of the worst presidents in United States history but also not even worthy of a comparison with our wartime leader who rescued our nation in its darkest hour".
Donald Trump meeting with Queen Elizabeth II is one of those things you just need to see to believe, and even then, it still looks photoshopped.
There will be no carriage procession and no opulent state banquet, and Buckingham Palace said no other members of the royal family will call in to Windsor to meet the billionaire-turned-politician.
U.S. President Donald Trump met Queen Elizabeth for the first time on Friday and got acquainted over tea.
However, the meeting with Queen Elizabeth on his first visit to Britain as U.S. president still seems to rankle.
The Queen, 92, stood at the dais as she awaited the Trumps, who each shook her hand.
Trump's motorcade pulled up to Windsor Castle in the afternoon. "Mary Jo Jacobi, a former aide to President Ronald Reagan, told CNBC's "Squawk Box Europe", that "[Trump] has already won, he is meeting with the queen and his lifelong wish has been meeting with the Her Majesty so from his personal perspective it is a win already". Trump's mother was born in Scotland. Oh dear, she just looked at her watch.
He added: "I think you are losing your culture".
Trump's previous comments about the royals have been less than tactful, including boasting that he would have slept with Princess Diana "without even hesitation", and saying "who wouldn't" photograph Kate, the duchess of Cambridge and Prince William's wife, topless.