I think there's some confusion as to BA's seniority list system.
When you join you are given a number in date of joining order about 3300 probably at the moment, this will get changed annually as people retire/leave so next year you might be 3250 etc etc. This is the only thing that matters for bidding to other fleets, so long as you are out of freeze and your seniority number is high enough you will get a course. Your seniority in your current fleet isnt relevent for this. There are guys 50 from the bottom of the 747 FO list who have commands this year because A320 commands at LGW are down to 2540 ish.