BA don't operate any 767s out of Gatwick anymore. They moved all the 744/763 long hauls to Heathrow several years ago. The 763 at Manchester is for the 1 daily JFK
flight, which isn't even a mainline BA
operated flight. Still, I guess they could move a 772 for that packed Manchester-Glasgow rush hour flight.
The big picture answer is one simple word: synergy.
A306, A318, A319, A320, A321, A332, A333, A343, A346, A388, B722, B732, B733, B734, B735, B73G, B738, B742, B744, B752, B753, B762, B763, B764, B772, B773, B77W, B787-8, BAe-146, Cessna Something, DC-10, E175, E195, ERJ145, MD-11, MD-80, PA Something