And why do BA feel the need to cancel so many shorthaul flights?
Schedule shift.
If there are inbound delays of one hour, then all the inbounds land an hour late. Maybe get to stand 90 minutes late after waiting for the outbound to vacate the stand......the next outbound sector that a/c is operating then departs 90 mins late, and so on. There will come a point that BA will not be able to fit all their flights into the day, before the night noise period. Better to cancel early in the day and re-arrange and re-schedule to be in a relatively good position come nightfall.