OK, here are are a few:
1) in the US if you "go to college" that means you go to a university or other institution of higher education.
But, in the UK, many people use "go to college" to mean they go to an institution of further education (age 16-18).
2) And a "school" in the UK seldom refers to a university, however it sometimes is used to refer to the "college" that a 16-18 year old might go to. Or it might be a school.
3) We don't have "kindergarten", which is in the US lexicology due to the German influence. We just call it pre-school, reception class, year 1 (I think).
4) Our school "year" system and your "grade" system are out of step by a year if I remember correctly.
5) We overuse the word "cheers". An Ohio girl told me she found it quite amusing that we would effectively make a toast every time someone held a door open, handed you something, said goodbye, pretty much anything.
6) An American girl I knew at uni sat in a puddle of beer someone had spilled on a chair and loudly declared, in a crowded bar, "Oh my god, my f*nny is wet!"... her (american) friend suggested she should "go and stick it under the dryer"... I think I may have actually p*ssed myself laughing.
7) Your speed limits often end in a 5, but all of our official speed limits end in a 0. You can get other limits on private land, but you will never see e.g. a 5mph or 65mph speed limit on a public road.
8) Public school here means completely the opposite to public school in the US (look it up on wikipedia for an explanation!)
9) I am assured (by the aforementioned American girls from 6) that British people dance differently from American people. From clubbing in the UK and the USA I can only imagine that this is because we tend to get packed into the dancefloor much tighter. In the US you have room to shake things about and spin a bit.
10) You leave your beer alone whilst you go and dance. We take it with us. This is why we have no need to shake things and spin around a bit.
11) A US pint is a fluid pound (16 oz), but a UK one is 20. Yours is more logical, but us gets us more beer
12) We drink because we want to get drunk. I have heard that in many US bars, the bartender will throw you out if you get drunk. Not drunk and disorderly, just drunk. I don't get it.
Yet McDonalds will serve you supersize even if you are already fat.
13) We never had the whole "Freedom Fries" issue... because we call them chips! You probably knew that already, but the British chip is much thicker than a "french fry", and is much softer. Generally we put salt and vinegar on them, but ketchup, brown sauce, barbecue sauce, curry sauce and mayonnaise are all acceptable condiments for chips.