as the new Mixed Fleet Crew have no entitlement to union membership nor have they any entitlement to collective bargaining
The right to union membership is inalienable, it is a basic human right. MF crews can join any union they want to. There is no recognised union for MF and there is no collective bargaining arrangement yet in place for MF. This will come in due course as it is in the best interests of both BA and MF.
There is very little chance that BASSA will feature as the recognised union for MF, though this can be forced should enough members of MF join and vote for recognition.