Stakenife, if you replace the word "Seniority" with "Loyalty" and BA now have a master loyalty list reflecting employees dedication to the company over the last few decades, with their "Loyalty" number reflecting time served, I think you'll find it is entirely ALLOWABLE under current employment law, and a legal challenge would be dubious at best.
BA's MSL, or MLL if you prefer,doesn't discriminate on age as we have all age groups at every stage of flying within BA. What legal basis would you have for challenging it?