Inspired by @solena
  1. All set
    This might be more of a New England thing but when your waiter asks you if you'd like another drink/dessert/a cheese plate 'all set' works great. English people are all 'Oh no thank you, that was lovely, I'm full now!'
  2. Heads up
    Genius American English phrase. The closest equivalent in English English would be 'Warning!' but that also indicates something sinister. So 'I want to let you into a little secret' is probably closest but uses a lot more words.
  3. Pants
    Americans are referring to the item of clothing covering one's legs. English people are referring to undergarments or a really bad time. "Last night was really pants" US pants would be trousers in the UK
  4. Costume
    This depends greatly on context but Americans likely to use around Halloween "Are you excited for the costume party?" and Brits likely to use at the beach "You didn't forget your costume did you?" referring to bathing costume (US swimsuit)
  5. Suspenders
    Likely to make an English person giggle. US suspenders (elastic things that hold pants/trousers up) are known as braces in the UK. Mention 'suspenders' to the average English guy and he'll think of US garters holding up stockings
  6. Jumper
    In America, likely to mean a person jumping off a building or an item of clothing the English would call a pinafore. In the UK, it's a cozy sweater.
  7. Restroom
    In America, this is the polite term for the room with the toilet. But this isn't really a word in the UK and folks will probably tell you there isn't anywhere you can rest or sleep available if you ask for it. Instead you should refer to it as the toilet or the WC. Even "bathroom" is confusing as that's the room where the bath is in England, not where the toilet is.
    Suggested by @eatthelove