Check out seat map sites like seatguru.com. Seats themselves vary, even within one airline, depending on the exact type of plane you are flying. For a long flight, particularly one overnight, that extra inch of seat width or leg room can make a big difference. As for things like meals and such, I tend to find them so inconsistent it is hard to say one is consistently better than another.

One thing to think about, though, is the time. A few airlines offer morning flights to London which get in in the late evening. depending on your specific plans, this may be a great thing - no overnight flight and jet lag, or terrible - you arrive too late to really do anything and it is harder to get back to the city.