You may want to refer back a few years when BA was looking to make some pilots redundant (2009). BA Management went to great pains to point out that Last In First Out was NOT a valid way of determining who gets to stay. This is backed up by a court decision between Rolls Royce and Unite Union. The outcome of that was that although 'seniority' / 'time in the company' could be used as a factor for choosing redundancies it does NOT have to be the only or even main variable when deciding. BA at the time maintained that each fleet could be viewed as a different entity and pilots could be made redundant off each fleet independently of the other fleets if that fleet was being downsized with seniority only counted when it came down to picking between two pilots on the same fleet and in the same rank.
So unfortunately when it comes to BMI all BA need to do is make geographical location the main factor when deciding who to let go. Legally they are quite entitled to do this, it may not sit well but they can do it.
So you are correct, seniority does not help in this situation.