Basically then, if I have understood you correctly, the Professional Aviator Spine is the key for aircrew to stay in without promotion - as understandably not everybody wants to be the Chief of the Air Staff - until they are 55, but they can still be promoted if they choose to Sqn Ldr, Wg Cdr and beyond?
Remembering of course that it is a two way choice, and the RAF has to offer you the PAS terms - otherwise, if you are not promoted by the cut-off age (was 38 in my day) then you are out. As for promotion...if you have not already been picked up for promotion by age 38 then your chances of being picked up after you are PAS are fairly slender unless there is a considerable shortage.