The twin seats on the 77W are quite decent, as well as the twins on the upper deck of the 2-class A380, don't think either flies to LHR, there might be on some flights from LGW. This is in terms of seats other than the exit rows that might offer some better leg room, beyond that it's pretty much all the same, I'd rather monitor the seat map closely (including up to hours before departure) and move around in order to keep an empty seat next to you if possible