I've read that LHR went down to 6 movements per hour at one stage... so it's hardly surprising that BA was the most affected.
Or, to put it another way, if BA had attempted to operate their published shorthaul schedule with that amount runway capacity, an A320 due to operate, say, 3 Belfast rotations would be running around 36 hours late by the end of the day,
Virgin 15 scheduled, 15 operated, zero cancelled
It will be a different story when it snows in January 2014 ...