Things not to say when visiting Great Britain:
"There's no U in 'color!'"
Or anything else that basically says that the UK is 'doing it wrong.'
"Gimme a Budweiser."
Seriously, you're going to the UK and you're not going to try the local favorite beers?
"I vill note buy thees recorrrd; eet eez scratched."
Quoting Monty Python will mark you as not only American, but also a hopeless nerd.
Ditto, but substitute Harry Potter.
You're not British, and no Briton is going to believe that you are, no matter how 'good' your friends back home think your English accent is.