I worked in London from 01 - 04. I can give you some tidbits about the place.
I am in finance and worked as a trader for one of the banks in Canary Wharf after working in NYC for 7 years. London is definitely different and here are some things to think about.
Career: Leaving NYC is leaving the mothership. Try to negotiate up front when you get to come back and get it in writing if you can. Once you become an "overseas guy" it is tough to get transferred back. They'll say "Sure we'll bring you back after a few years" Be skeptical of bland vague assurances. If you are paid in US$ and you think the dollar will continue to fall, you can hedge your fx exposure using a spread betting place like City Index.
Cultural: It is a nice change. The brits believe in taking 2 weeks vacation at once. Nice. They also like to drink. If there are 4 or 5 of you on the desk and you go out for a "quick pint" after work, everyone in the group will have to buy a round, which equates to roughly 6 - 7 pints of what you are used to over here. Since the pubs close at 11:00 they don't like to waste valuable drinking time having dinner which means you will end up eating chicken vindaloo at some dive curry house before you go home. 7 pints of Stella on Thursday night + curry at midnight = one nasty hangover Friday morning. Luckily, the entire continent goes out drinking Thursday night so no one expects all that much out of anyone on Friday mornings. If you work in the docklands, thursday and friday nights are good. Essex birds are easy... buy them a couple tart pops, tap them on the head and their knickers go down.
The downside culturally: "Service" is an English word invented by Americans. Get used to lackadaisical service. The default answer is "no". Getting mad is useless - you will find yourself in English passive-aggressive hell. Do not, do not, do not close your American bank account. Get a UK bank account, but make sure you have a US account with a decent amount of cash in it. Get used to your US credit card thinking everything you do is fraud. You will find yourself having to "take a call" a lot when the dinner check arrives.
Expenses: London is the most expensive city on earth. I think I was paying 1100 pounds a week for a mews house in Central London and that was at 1.40 to the dollar. My experience was to more or less look at what you pay for something in the US, change the currency symbol and thats what you will pay in London.
I lived in Bayswater in Central London. Bayswater, Notting Hill, Victoria, Knightsbridge, Maida Vale, Marlyebone, South Ken, are all great choices. Hyde Park is a great place to run. St John's Wood and Hampstead are good if you have a family. The docklands are good if you work at Canary Wharf.
Finally, be sure to travel a lot when you are over there. You can leave work after the market closes and make 9:00 dinner reservations in Paris.