lessons I've learned working in London for the past 8 years.

When you are asked "Good weekend?", they actually don't want the details of your weekend.
Don't get too excited when somebody tells you something is "Brilliant." Everything is brilliant.
Pants = Underwear. So don't say, "I've been lazy and have been wearing these pants all week". Not good. The correct word is trousers.
Don't be overly nice to a Brit when you meet them they will wonder what you want from them.
Also, don't tell everyone your life story when you first meet them.
You don't do Europe.
Ibiza is prounouced IbiTHA.
Tea is dinner.
Marmite is nasty.
Don't get all hurt when somebody disagrees/debates with you at work. It's just part of the culture.
Getting a coworker to come over for dinner may take 10-15 years.
They butter both sides of sandwich. Still can't get used to it.
It's holiday not vacation.
They take their holidays very seriously. You will not impress a Brit by saying, I haven't been on holiday in 6 years.
Muppet = Idiot.
It's very annoying when a Brit mimics an American accent and says, "Awesome." It seems to amuse them though so don't say anything.
Don't talk about America all the time. It's annoying.
Don't develop a fake English accent. It's also annoying
Brits don't wear undershirts.
Never, ever wear your blackberry or smart phone on your belt. That is equivalent to high treason.
Get used to army time and DD/MM/YYYY
Say no to Khakis.
First stop when you get off the plane is to TM Lewin for the 4 for a 100 deal. Paul Smith and Thomas Pink come later on.
Embrace cuff links.