I actually work for United*, so let me add a couple of things:
OnePass is merging into MileagePlus at the end of the year, but since the programs were aligned as part of Star Alliance before, nothing much changes and your miles will carry over from one to the other.
Nearly all our ex-United international aircraft (all 767, all 747, and most 777) have individual video-on-demand screens at each seat. In Economy, you get basic access for free and premium content from HBO and some new-releases for a flat fee per flight. As I work on the ex-United side, I don't know much about the ex-Continental side except that their planes are very new, so they shouldn't disappoint.
You will be fed, and you can request a special meal if you have dietary restrictions (or you would rather have vegetarian or low-sodium for whatever reason.)
For your seating request, be sure to ask at the airport as they do hold back a certain number of seats so they can accommodate last-minute families and groups. If you were flying a United-operated flight, I could look at it for you, but I don't have access to the Continental systems yet.
I used to work at San Diego airport, and I've flown to Heathrow pretty extensively, I'd be happy to answer your or anyone else's questions about air travel.
Have a great trip!
ian
* I work for United, but I speak for myself alone and not on their behalf.