There's no real "announcement" you have to make that you're moving away from the US. But there are little things to do that you would do in any move (even if it's just across the street), such as change your address, stop your paper, notify whoever needs to know, etc. Even though I felt I'd thought of everything, when I went by the US Embassy to take care of little things, like changing my name on my Social Security card, I discovered a whole bunch more services they had that I was unaware of. Questions answered for driver's licenses, taxes, etc., forms that could be picked up and other helpful things.
If you want to keep voting, let your local voting place know your new address; they will send you absentee ballots to use. Certainly let Social Security and the Internal Revenue know where you are. Also, inform the US Embassy in London. You can try to change your driver's license address, but they'll probably just laugh it off 'yeah surrrrrre' like they did to me.
You've got a good four months to think of all this stuff. Note what comes in the mail to you during this time. Either cancel the service, or change your address (if you know the new one). Look up checks in your checkbook that you wrote for the past year. There could be an annual subscription or membership that you may have forgotten about. Is your bank/credit card company willing to send your statements overseas? Frankly, I have US bank and credit card accounts that I have found a huge help, even though I live over here. If your current accounts don't handle UK addresses, then you may want to open up an account where they do. Wachovia and American Express are companies I know of. You might even want to open up an AOL account; you can certainly "bring" that with you in your move just about anyplace. Just because you're moving to some place, don't close the door behind you.
Anyway, a long explanation just to really tell you that there aren't any "exit" forms. But you are moving, so take the next four months and think about all the things you have to do to make your move easier while you're still there, remembering it's not all that easy to run back and handle anything that may come up.