Grads have been paid more in training for as long as I can remember - and that's thirty plus years. All three services gave them and probably still give them three years seniority over a first year Midshipman/Officer Cadet. Some non-grads were able to "earn" seniority before entry if they had quals., and I joined with an ex civil serpent who'd been an EO, and so claimed a year on me, a mere 18 y/o who'd spent his six months between school and BRNC working in M&S (lovely girls to work with, mind you)
Don't forget that about 25 years ago the RAF was pushing for an all-graduate officer entry - from which it seems to have rowed back since it discovered that not so many grads were good aircrew stude material.