you only get seniority for going to uni until age 21, this is prob to prevent loafing gits who hav 3 gap years then do a 1st degree in underwater basket weaving joining as lt with 3 years seniority.
Seniority is actually awarded for doing the job, not preparing for it, and while I agree with the current system, which is a lot fairer than the army equivalent, I don't think it needs to be taken any further than it is already.
Even UCEs (joined up, BRNC, then uni) who did Masters degrees marked time at SLt +2 seniority so they weren't promoted earlier than anyone else.