4468, BA are and have been offering contracts to new starters on the new pay point deal (i know of people starting next month for example). If at the same time they are making people redundant without options then this I think puts them in a legally difficult situation if they do not offer alternative employment at LHR.
Isn't the point that BA aren't doing anything at all at this point.
IAG are giving notice that roles
may be at risk of redundancy. It seems this makes all the difference as any surplus will be dispensed with before BA gets the keys to BMI, sidestepping the legal issues you mention.
Like other posters, my personal feeling is that this isn't the end of the story yet. It may well transpire that some roles can be, after negotiation, saved by relocating to LHR. Time will tell.