They better think this through. For example, they need to consider what if he becomes US president. He would then be seen like second George Washington.
Although it probably won’t occur...I’m of the belief if they were that ‘stupid’...a President Trump would note that if he wasn’t welcomed....neither were US troops in the UK, and he’d remove them in a rapid fashion, and let the UK know the score. Some French would applaud.....as would some Germans.
The comedy of this would be that Lady GaGa gets a free-ticket to enter the UK, and Trump would be forbidden. Something about that logic...would be a great comedy for the Monty Python crew to do a skit explaining this to a normal Brit at a pub via a government official.