Well if you heard it at the BASSA meeting it must be true!!
The Bridlington agreement which stops unions poaching each others members is not enforceable and would not be relevant in this instance.
Bridlington forbids a union from actively recruiting existing members of another union. Mixed fleet crew are not members of Unite/BASSA and the GMB are quite entitled to recruit them.
As I have stated before the GMB has had recognition rights for cabin crew in the past but just didn't bother to utilise it.
I will consider eating my old BOAC cap if it turns out not to be true that GMB have recognition rights for MF.