Well, let's get the stereotypical confusions out of the way... "football" is what you call "soccer", and "fags" are cigarettes.
BBC America has some stuff to make our TV programmes seem less confusing:
http://www.bbcamerica.com/britain/all_about_britain.jsp
You may have noticed we spell things differently. Be prepared for unexpected uses of the letter "u" as part of our colourful language.
Politically, we're very much to the left of the US. The Guardian (affectionately known as the Grauniad because of its reputation for typos) would make the average Republican's head explode if they read it, and as such is considered a weapon of mass destruction in several states. The political left here is red and the right is blue, I gather you have this backwards in the US.
If you want to fit in, then partake in some "vaguely complaining about stuff but not really doing much about it". Favourite targets seem to be telcos - nobody really likes BT, but they put up with it because ntl's service is even worse. You should be OK, though, I think London is mostly covered by Telewest.
We have four and a half terrestrial TV stations (five doesn't really count, as none of the programmes it shows are any good except CSI). Two of these are run by the BBC, with no adverts. This will cause a strange thing to happen - sometimes programmes reviewing things will not give every single product a glowing review in case the adverts get pulled.
We don't really like guns much either, and our politicians are actually allowed to pass laws against them, so they do.
You may know that your french fries are our chips, but that's not the whole story. If you go to a fish & chip shop, you will not be served the thin stuff like the burger chains sell, you'll get thick cut chips every time. It's the same story if you order chips with a meal.
We have crisps as well (US chips). The most popular brand is Walkers, but the best IMO is Seabrook. Last time I went to London they weren't widely available, but they are much easier to find up north.
As for driving tips: If you're tempted to buy a Chevrolet, don't. In the UK, these are in fact Daewoo cars with a different badge on. Since petrol (we don't call it gas) is so expensive here (you will have noticed this unless you didn't realise the price was in litres and the same figures in pounds is a higher price than in dollars), you should avoid as large a car as you may use in the US. If you're in London, there's the congestion charge to think of anyway, but if I lived there the tempting option would be a Smart.
I'd recommend reading some Douglas Adams books, and listening to Radio Four (especially the comedies). This should help you to tap into our minds.
Also, get used to waiting ages for new films, TV shows, console games to come out here. Excuse me while I nip off to grumble about waiting another two months for Animal Crossing DS...