But Lucy Lawson, an American living in Britain, said that while she opposed Mr. Trump’s policies, she considered the protest infantile.
“Why are people going down to his level?” she asked.
Ms. Lawson asked one of the organizers why they launched the balloon, knowing that Mr. Trump would not be in London.
“It’s going to swamp his Twitter feed,” Mr. Cottrell said. “There’s no way he doesn’t see this.”
Meeting the queen? There are rules
If Mr. Trump was a bull in a china shop this week with NATO and Mrs. May, he will presumably be a bit more restrained when he and his wife, Melania, meet Queen Elizabeth II at Windsor Castle.
Mrs. Trump was briefed on royal protocol ahead of the visit with the queen, her spokeswoman said. For first ladies, that would typically include briefings on how to curtsy, how to sit properly, and teatime etiquette.
It was unclear what preparation, if any, was given to the president.
Myka Meier, an expert in protocol and etiquette at Beaumont Etiquette, said in an email that there was no official code of behavior for meeting the queen, though tradition frowns on turning one’s back on the monarch or sitting before she does.
But there is one rule, she said: No touching.
“In the event that the queen extends a handshake to her guests, they may then reciprocate the gesture,” Ms. Meier wrote.
A person touching a royal can make headlines, as Michelle Obama did when she touched the queen in 2009. Jimmy Carter scandalized Britain by kissing the queen’s mother — on the lips, no less.
Grant Harrold, a former butler for several royal family members, advised keeping the conversation light — when in doubt, limit it to weather and the queen’s corgis, he said in an email. He also had advice for Mr. Trump on shaking hands.
“No hand clasps or ongoing pumps,” Mr. Harrold wrote, “as we are meeting the Queen not pumping up a bicycle tire.” — Katie Rogers