1) Hit seatguru.com and look up the plane you will be on. You'll be able to find out if you'll have a power port at your seat, and also learn about any issues with the seat you are on. My advice is to upgrade to Business Class if you can. Far more comfortable.
2) Bring your laptop. Your wifi will work out of the box, no adjustment necessary (I don't even think there are region codes on wireless, but I could be wrong), but pickup a travel plug. Your laptop's power brick should be autoswitching and will handle the 220V/50Hz power now problem, but check to be sure. Don't buy a new cord unless you are actually planning to move there.
3) Will you be traveling alone or with someone else? If you are with someone, get each of you a Pre-Paid phone. If you already have a GSM-Phone, check if it's a tri- or quad-band phone. You may be able to make do with just buying a Pre-Paid SIM and swapping them in. Either way, you can find many places to pick them up, including Vending Machines.
4) In the states, it seems like everyone takes Credit/Debit cards and nobody cares if it's Visa or Mastercard. Not so over here. I've seen quite a few ATMs that VISA only, so be sure to check. It helps to carry a few quid with you, just in case. FYI, Quid is slang for Pound(s).
5) Someone else said it, but remember to LOOK RIGHT when crossing the street. We usually look left first, and then walk into the street as we look right. This can be deadly in the UK.
6) Most of us are aware that not everyone loves us Yanks. The UK is pretty safe for Americans to travel, but be aware just the same. The movie Taken, while certainly overly-dramatized, makes a good point. Nobody thinks that it will be them. You don't need to be paranoid, just cautious.
7) Be nice. Try to prove the American stereotype wrong.
Lastly, 8) HAVE FUN. The place is kewl and they have some great beer. Definitely hit the pubs, even if you don't drink. They're an experience.