And British hosts, please don't offer to knock your American houseguests up in the morning, or any other time.

Fascinating fact: the phrase "knock up," as in awaken by knocking, comes from the industrial revolution, after people were having to rise at a certain hour for the first time in history, but before the invention of the alarm clock. The "knocker-upper" was a person who went round with a long wooden pole, knocking on the windows to get people out of bed for their factory shifts.

https://en.wikipedia.org/wiki/Knocker-up

Unless you're attractive. Then they might appreciate the offer.

My British in-laws always have a good laugh when we go by the "Frugal Fannies" store in the States.

Reading that in British English: coffee, meet keyboard.

My favo(u)rite ambiguous sentence: I'm mad about the flat. What do you think it means? ;-)

Even better is "can I bum a fag?", which wins odd looks on both sides of the pond.