sorry, seniority is NOT recognised in law...............period. There is NO obligation to merge seniority lists
Isn't it the other way round?
Because seniority isn't recognized in law, you SHOULD merge the seniority lists because you can't just stick people on the bottom of the list?
seniority cannot even be used for redundancy legally due to age discrimination laws.
This isn't quite correct either, because "seniority" is not necessarily equivalent to "age". The age distribution, even in BA's seniority list, is not monotonically decreasing.