Groovy Mother points to a Wikipedia list of British English words not used in American English.

Since moving to Redmond, I've had to learn some of these the hard way.

Recent examples of words I've used in conversation and got back a blank stare:

• Fortnight (a period of 14 days (and nights) or two weeks)
• F**kwit (idiot)
• Aubergine (eggplant)
• A*se (ass)
• Blag ( to obtain or achieve by deception)
• Boot (trunk of car)
• Git (mild derogatory)
• Spiffing (very good)
• Zebra crossing (pedestrian street crossing)
• Bugger off (go away)
• Cock-up (mistake)
• Yob (lout)
• W*nker (generic insult)
• Spawny (lucky)
• Bloke (guy)
• Marrow (squash as in the vegetable)
• Slaphead (bald man)
• Cul-de-Sac (dead end, french word in fact)
• to go pear-shaped (to go wrong)

For illustrative purposes, here's what sentence or two might look like using the above.

"Oi, F**kwit.  I've just blagged this spiffing aubergine off this bloke.  Now, I'm going to meet the w*nker again in a fortnight by the totally unused zebra crossing by the cul-de-sac to sell him back a marrow.  Got it, git? Now, in the meantime, I need you, slaphead, to keep it safe and sound in your boot, 'cause the spawny a*se might realise he's just cocked-up and try to get it back. By the way, that's 'realise' with an 's'.  And it'll all go pear-shaped then. Anyway, here it is, now bugger off."