As one with an aviation background but no involvement and an active trade unionist in another field may I throw this bit in the ring?
UK employment law is robust, those employed from outside UK must jump some pretty high hurdles, in the case of Canadians without required dual nationality or work visas they would not be allowed to be employed by a UK employer, those with the status to be employed by a UK employer have equal rights to compete for positions. NO union can change and it is a waste of time berating any union for doing nothing, it is HMG they must move.
In short, neither BALPA nor IPA have any power to prevent the situation posed, in the same way as they cannot prevent any other legal practice such as training schemes run by OAA etc.
Sorry chaps, that's the way it is.