General

Flex in England is an extension cord in the US.

Lift in England is an elevator in the US.

Bonnet in England is hood in the US.

Fanny pack is the one that gets me.

I can add a few...colloquial sayings from where I grew up.

Aderkaderr: A word to express how nonsensical a previous statement is.

Later Days=See you around or Check you later.

Lemmelow: Leave me alone

Jeetyet: Did you eat yet? *probably the most relevent to this site lol*

I got some funny looks when I lived in the US for a bit and asked folk if I could "bum a fag" off of them.

Along the same lines as Heroin Chic's post.

Doohicky: General term for an object.

And a couple of others that just now came to mind.

Hey you with the face: A statement used to catch the attention of someone who's name you can't recall.

That girl with the hair: a statement used to talk about a girl who's name you can't recall.

I'll think I'll do one (quite apply) for food, so in the ol' UK:
fries are chips
chips are crisps
cookies are biscuits (except cookies, which are still cookies)
candy tends to be sweets (or chocolate, if it is)
a sarnie is a sandwich - bacon sarnie being a favourite
a fry up is a breakfast meal consisting of fired food. Typically including bacon, egg, bread, sausage, hash brown, mushroom, beans, (won't necessarily include all variables, but they can be changed at will)

Boosting this and to say that a fry up is also the same as a 'full english,' when American's would call that an early morning abomination. Especially if the plate has some black pudding on it!

Others (from UK to USA English):
Pudding = dessert
Butty = sandwich
Chemist/s = pharmacy/ist
Newsagent = corner or liquor store
Way out = exit
Torch = flashlight
How long is a piece of string? = Who knows? / I don't know

I'm sure I could think of more, as I find that being an American in England means I'm still learning new words and phrases regularly. Even after living here a year and a half!

well there's tea the drink or tea as an alternative work for dinner.

"I'm going to have a cup of tea with my tea"

and as for the 5th floor, I've no idea.
My only guess is if they call when the enter the shop (store) the first floor. Whereas we call it the ground floor, like a 0 floor, and the counting only begins when you start going up in the lift (elevator)

A couple more Aussie terms:

arvo = afternoon
durry = cigarette
durry muncher = cigarette smoker
ranga = redhead
singlet = vest/tank top
mobile phone = cell phone
garbage bin = trash can

Also we just use the word 'prawn' for types of shrimp here in general, and yes, we do barbecue prawns sometimes though we mostly do steak and sausages I've found.

A Bluebell in Scotland, is not the same plant as a bluebell in England. However, a rose by any other name would smell as sweet.