There are a couple of possible explanations for this.
It is quite likely to be a shortage of cabin crew rather than pilots, and some of the junior managers in customer services don't know enough about airlines to differentiate between 'Flight Deck Crew' (i.e. Pilots/Flt. Engineers) and 'Cabin Crew'. It is not a huge mental leap to believe that someone could see the expression 'Flight Crew' as meaning any crew who work on board an Aircraft whilst it is flying. So not necessarily a shortage of Pilots IMHO. Expect to keep waiting for BA to begin pilot recruitment.
Now, whichever side of the locked flight deck door the shortage lay (and from where I sit there is a shortage of staff both sides), the difficulty that BA has is that is still has to achieve the MPE (Manpower Equivalent) reductions it promised the city in its BRS (Business Response Scheme) following FSS (Future Size and Shape).
Unfortunately, they still need to cut more staff but seem to be doing it from frontline staff rather than the mountain of beancounters and other 'less productive' staff in Waterworld etc. Additionally, a lot of the cabin crew and check-in staff who took unpaid leave a year or more ago are failing to return having found something else to do in the meantime.
The final complication regarding any imminent pilot recruitment is that it opens up a whole can of worms with respect to the closure of NAPS and the existing pilots recognising that if we accept its clousre to new entrant pilots then we probably won't even collect our own pensions when the time comes (i.e. it will be wound up completely). There are currently still lots of Cadets who were recruited before NAPS was closed to new entrants coming on line at the moment but it won't be long before the shortage of pilots becomes acute though, as all fleets are complaining of being badly underestablished at the moment with the CAP is very high on bidline fleets and SHAG pilots permenantly shagged out.
As to why they are continuing to offer pilots unpaid leave, I simply cannot imagine. BA is a bizzare place to work!
On balance the above cancellation is more likely to be a CC shortage than a pilot one. CC have high sickness levels at the moment as the company is currently denying them leave (to which they are contractually entitled) due to the acute shortage, so the only way they can get a day off for something important is to go sick, thus exacerbating the problem. Catch 22 n'est pas?