JW95
BA's Canadian services are poor. YYC gone, YUL a mere B788, YYZ twice daily and YVR daily. WS want to better compete against Air Canada and to fill the front cabin easier, they want to move to LHR. They can't match Air Canada but they can compete better with BA to Western Canada and YYZ. Your point would be a good one if we were talking about Air Transat, who did try LHR for a few years and are genuinely better fitted and do well at LGW.
Talk of a "Gatwick catchment area" is not helpful IMHO. It's a Canada-London market predominantly here, and for the segment WS seek to grow, LHR makes more sense. That's why they keep trying to leave LGW!