The Direct.gov site that Desk Jockey quotes goes on to say
Other unfavourable treatment for a trade union membership reason
Treating you unfavourably includes, for example, refusing you promotion or training opportunities, or withholding a pay increase.
If you are a trade union member your employer must not treat you unfairly in order to deter you from:
- joining a trade union
- taking part in its activities
- making use of the services it provides to its members
- leaving it
which seems to cover the point. I wonder whether the truth of the matter is that, whilst an employee of the new Mixed Fleet could be a member of BASSA / Unite, there will be no union recognition and therefore no collective bargaining rights.