The RAF C-17s operate to the Global Fleet concept, which keeps them supposedly in step with the rest of the world. The slight irony in that is that it actually puts us ahead of most of the USAF jets - I have no idea how the 'queue' (or 'line' for our Stateside buddies!) works for the upgrades. The fact is, we get each and every one of them.
What you (KenV) have to NB is that us "micro-fleets" of 4-10 jets work quite differently to the USAF with their 220+ frames. To put up a training jet takes a significant portion of our frames (1/6 on the line) and thus keeping the lowest possible training burden is paramount...to the point where, yes, it can even drive capability. Even synthetics suffer this same issue.
That's sadly just how it is operating in such relatively small numbers...