Not a Pilot but just a Pax, however having been through this process over the last 2 months with lawyers giving me daily migraines, the legislation is full of really useful stuff (not) like selection must be 'fair' - however the agreed outcome was that selection criteria needs to be able to survive a challenge at an employment tribunal and should consist of 'hard' and 'soft' criteria
Hard being
Historic capability / rating to do the job
Amount of self certified sick leave over previous rolling 12 months
Current 'scoring' for the exact role (In your world you couldnt replace a 2 year A320 pilot with a 15 year 747 pilot who needs re-training)
Soft including
General perception of managers and seniors as a company asset
Peers comments in historic appraisals / ratings
Length of service on its own the company lawyers believed could not be defended at tribunal i.e its unfair dismissal to use this criteria