Wrong, in the UK every employee is entitled to be a member of a trade union. If represented by a trade union a member will be represented through collective bargaining once the employer recognises the union through voluntary or statutory means. At least one of the offers put to the cabin crew included recognition of MF crew as a bargaining group to be represented by UNITE so I would imagine that would be BA's preference going forward.
I suggest you read the above post especially the answer from BA which states:
There are no collective agreements that will apply to you under the terms of your contract of employment unless you are, at some future date, told that there is a collective agreement that expressly applies to you.