Most of the 763s going to LHR are 2-2-1 seating, so 2A is a window seat on a row of 2. The best seat on a 2-2-1 763 is 4K because it reclines almost flat. Otherwise it doesn't make much difference.

On a 333 1A is also a window seat and a bulkhead. It wouldn't be my choice. Also, I think they usually do the meal service from the back on the 333s, so rows 3 or 4 are preferable if you want faster service. The last couple of rows are in the mini-J cabin and seem to have a slightly reduced pitch, so you might want to avoid them.

On a 319/320, 1A also a bulkhead, so you're probably much better off taking a couple of rows back. The seats are all the same.