Hand Solo,
thanks for the lesson in UK law. I can assure you that the principal of free contract negotiation is valid in just about every country of the civilized world, which I'm sure the UK is a part of.
Just like you correctly mentioned, it depends on the circumstances, HOW the agreement was reached, particularly on the positive intention of both parties. Not knowing all the facts, but reading through the posts, this agreement seemed to be reached under rather dubious circumstances..
There is many "if's" and "when's", but it appears, that a smart lawyer could have fun tearing BALPA apart...