If BMI was bust and BA rescues it, that would be one thing, but just buying out a competitor surely BA should respect the internal structure of their purchase.
To all intents and purposes bmi is bust and there'll be little, if any, of it's internal structure left after BA have finished with it. As Ollie said, seniority has very little legal strength. If BA were halving the LHR operation there might be scope to use seniority as a factor in the redundancy matrix, but as they are closing bases in their entirety then there is no requirement for a matrix to choose between staff as they are
all going. Unfortunately I think we are seeing the first steps towards a new paradigm in airline management in the UK in which seniority counts for little. This is the way it's going to be from now on.