Over 2 years in London and I am still learning new differences in the same language! Brits continue to correct me when I say the wrong thing or even better, they act like they have no clue what I am talking about! A few examples...
Pavement: The sidewalk.
Wally: A foolish or silly person. Similar to an idiot.
Jumper: A sweater. I have used this one before but I am constantly harassed for not using jumper. Try as I might I just think it sounds stupid to say jumper.
Have a go: When you are angry or upset with someone you 'have a go' at them or tell them off.
Pissed: Drunk. This can get you in trouble. If you want to say you are angry you must include off.
The shits: to be upset. So this is more of an Australian slang but I hear it all the time and giggle uncontrollably. Used in a sentence, "you know what gives me the shits? British people making fun of me for saying cell phone."
Mobile: Cell phone. The one and only acceptable term for this device. Pronounced with a hard i as in exile.
Shed load: Replace the ed with it and you should get the idea. It means a lot. This is Chris guilty giggle term. He hears it used in formal presentations on a regular basis.