this is not France, so BALPA cannot call for e.g. a wildcat strike or selective withdrawal of training to stymie the company’s recruiting plans?
No but they could call a ballot and start the process of (perfectly legal) industrial action if they wanted. Doing nothing is a not a result of UK strike law, it's a decision made by pilots and their union representation. Tbh the reality is an 18k type rating is probably not top of the list of grievances, they'll still have cadets queuing round the block. But yet again it's an example of new entrant terms being lowered with seemingly no protest.
I don't work in aviation (unfortunately) but I know that if my company decided to make all new graduates pay upfront for their own training, they'd a) have no applicants and b) cause a riot amongst current staff. Why the same logic doesn't apply in the airlines is beyond me.