What they are eluding to, is what will happen when a fleet is downsized/removed. Can the employer then make the outgoing fleet pilots redundant and directly recruit pilots on to the new fleet. Thus removing the more expensive (based on years of service) existing pilots, to recruit cheaper direct entry pilots.
No. Employment law is quite clear about this, you cannot make some one redundant and then replace them, the argument about fleets is a red herring in this case.
BMI have an agreement based on LIFO and it's pretty clear what BA have decided is legal regarding that.
It may be a minor point but BA have not made these decisions regarding BMI, it is IAG. BA will not have control until BA takes over BMI. So please castigate the correct organisation.