Betty Girl, I think it is a consequence of the collective bargaining agreement that those outside BASSA have been given the opportunity to go part time.
Those who are still in the union have delegated their bargaining rights, in toto, to BASSA and so long as BASSA have an agreed mechanism for transfers, and so long as they are in dispute with their employer, then BA do not have an obligation to go through the mechanism for BASSA members.
It s the same for the Disruption Agreement - BA might do the decent thing and inform BASSA that they are implementing it (assuming there are any channels of communication still open), but do not seek the agreement of the union before doing so.