BALPA once helped me take an airline (long since defunct) to an Industrial Tribunal. They sent two lawyers and won the case easily. So I wouldn't agree that BALPA only look after BA, or that there is cronyism. As the guy who came up from London to help me said, they have finite resources and they have to pick their fights. They may seem expensive, but their membership base is relatively small for a union. In my case, the figure we achieved at the Tribunal covered my subs for 20 years.
Having said all that, nobody should automatically expect the cavalry to charge in every time there are redundancies. It sounds to me as though the circumstances of the first post would be difficult for BALPA to effectively contest. Such, unfortunately sometimes, is life.
With regard to contracts, they may change as the law does, but the employer is obliged to notify their staff of ANY change to their contract. Few do, in my experience.