Sites like SeatGuru will show you the seat width and seat pitch on the aircraft planned for the flight. (Seat pitch isn't a measure of leg room, but it's close. It's a measure between any point on a seat and the same point on the next seat.) Airlines do occasionally need to switch aircraft and that, of course, can throw off seat assignments.
The economics of the industry drive every airline to put as many people on board each flight as possible. So in any given aircraft the cheap seats are going to be the most restrictive. Tall and thin folks might look for more seat pitch, while shorter and not-so-thin folks might look for more seat width.
I'm 6'3" and unless I buy an upgrade my knees invariably press into the back of the seat in front of me in economy. If that seat's occupant reclines more than a fraction of an inch, they intrude into my space.
Also, a few daytime (leaving in the morning) flights to London are available from some east coast airports. All others are typically overnight flights. If you think sleeping while scrunched up in an uncomfortable seat might be an issue, maybe check out a morning departure.