They canx short haul because it causes less problems (relatively!) than diverting long hauls that are already 75% of the way through their flight time.
Secondly, the canx a lot because it give mores time to arrange alternatives for the pax. BA have to presume that the closure will be an hour and may be longer. If they canx two and a five minutes later a couple more and then a couple more - pax would say that they are not being given enough advance warning. Better to announce, say, ten cancellations and then maybe reinstate two.