Well, I'm at Oxford University, so I can tell you that Oxford's not really like the rest of the UK in many ways! Too many clever people around. ;-)
We (UK and Oxford!) DO drink quite a lot of tea, lots of booze (especially when a student), and we're much more sarcastic than Americans - so if someone says something mean, chances are they are being sarcastic, because British people tend to be very polite as a rule.
We're not as openly enthusiastic as Americans (that's one thing I love about America!) but on the other hand we are better at putting up with stuff and not complaining.
We eat with a knife in the right hand and fork in the left hand and use BOTH to eat with at the same time - if you do the American thing of cutting up your food and then just using the fork to eat, it will make you stick out a mile!!
Adult British people don't usually drink milk!
We walk around a LOT, and everywhere is made for walking.
The streets and houses are generally much smaller than in the US.
You usually can't get coffee/food 24 hours here - or anything else, except maybe a supermarket if you're very lucky.
Going down the pub seems to be our major hobby ...
Slang - hmm - chavs are like your trailor trash tho slightly different.
I'd suggest watching 'Little Britain' - the UK one not the American version - and 'The Office', again the UK one not the American one. They should give you quite good insights!