I came over here from California (more or less permanently) three years ago. It's been an interesting and amazing experience, and I'm sure you'll have a great time.
Some advice, for what it's worth:
Housing is very expensive, yes, especially in London and the southeast in general. Also, be prepared to have someplace that by American standards is very small. If your flat or house has a washer, don't expect it to have a dryer. Most people here dry their clothes the old fashioned way. :-)
Also prepare to be shocked by the price of many everyday items, all the way down the line from electronics to cans of Coke. It's the one thing upon which every American invariably fixates. It's best to try to stop thinking in terms of dollars and think in terms of pounds. It's the only way to stay sane!
Regarding language and culture, a friend of mine who is British once described speaking to Americans as "like playing a piano where some of the keys don't work," and the same is true in reverse. In some ways I think it's actually more difficult than traveling to or living in a country that speaks a completely different language, as in that case there's no assumption of cultural or linguistic knowledge. Be prepared to misunderstand anywhere betweeen 5% to 25% of what people say when you first arrive, be it unfamiliar words, words with completely different meanings (you don't want to know what "fanny" means here) or pop culture references. Don't be afraid to ask for an explanation or be asked for one, though Brits are generally more familiar with our lingo than we are with theirs (thanks to the Simpsons, amongst other things.)
I live in Cambridge, which is about sixty miles north of London. It takes about an hour to get here on the train. I'd advise against looking for jobs on Cambridge - not because it's difficult to find them, but because it's generally quite cliquey and difficult to meet new people relative to London (or just about any other place I've been in this country.)
Another thing you may want to investigate is low-cost airfares to the rest of Europe. It is quite inexpensive to fly from the UK to Italy, Spain, Germany, etc. Check out
http://www.skyscanner.com for fares. If you're going to be here for six months, it's a great opportunity to check out more of Europe for much less than you'd be able to if you were flying from the states. There's something particularly great about being able to take a long weekend in Tuscany without too much hassle. :-)
Anyway, I'll be happy to answer any other questions you might have from my perspective as an American who has lived here through some time. Good luck!