BA are top of the food chain and an employer of choice rather than necessity for a pilot. They can still afford to be fussy as they offer legacy airline conditions of employment, except for recent pension changes. A desirable main base with the ability to commute from other locations and a good fleet/route mix offer a decent lifestyle. No wonder there is very little turnover.
Any shortage will be felt lower down the chain by operators who took full advantage of previous pilot surpluses to drive down pay and conditions. Foreign airlines which previously offered desirable expat packages but continually eroded them and second level airlines with no respect for their employees are in the firing line.