So, why are about 50% of the LHR-ABZ-LHR flights over 30 minutes late.
My guess is that only certain flights are late, usually the later ones in the day due to the inbound aeroplane being delayed.
I used to do Aberdeen - London alot, and I recall the Early morning flight times were scheduled over 30 mins longer down to London than those returning in the evening - I put it down to the 20 minutes holding over the south downs before landing....
As for certain flights regularly being cancelled, I have also wondered if it was due to poor loads combined with a delay with the inbound aircraft meaning its easier to cancel than fly late and screw up even more flights further down the line for that aircraft.
It isnt just BA though, all the airlines seem to have this problem. the current "regular delay" for me is the last KLM AMS-BOD flight, usually due to the cumulative delays throughout the day of the inbound aeroplane.