This is only about fast jet pilot streaming.
I spent around seven years as SO1 Trg at HQ 1 Gp and, as such, attended (as the customer with full voting rights) every role disposal conference that I was in the country for during that time. I knew where the OCU slots were, where the front line need was and what the dilution rates, upward requirements and (from the posters) aspirations, dissatisfactions and onward Manning projections for each of the forces were.
Each and every training graduate's report summaries and recommendations were examined carefully and considered for the current slots available and, in many cases, against future needs for future slots. More often than not we would need to delve deeper and call a halt to read the relevant, individual sortie reports (with a degree of cross-referencing).
Let me just plug in a little personal thought at this point. If you grade aircraft types against pilot performance in training, you will consistently risk sending the lower end of your training system to the type that someone has decided is the safest place place to put (potentially) a disproportionate number of so called training risks. You dilute that force. You seriously degrade its capability and risk creating a lack of worthy supervisors, "Qs" and future leaders. That is madness.
So, the "needs of the service" is really the needs of each force with the bigger picture in mind. The Harrier force could not demand the best (as they saw them) guys from training on the grounds that there were handling demands, at the same time that the Tornado force could not be expected always to take the "others". I am not making any comment about folk that constituted that same committee before my time there (I was was also a member of it as a TWU flight commander many years earlier and cand attest to the same level of rigour then).
We smashed the "Harrier myth" by sending a range of pilots to them without completely shining training reports, we maintained the balance of skill levels between the forces, we addressed the needs of the new Typhoon OCU. And downstream success/failure/recource rates did not change significantly.
My job didn't finish there because the OCUs sat on my desk, so I then got to watch the trainees go through their conversion courses and their squadron work-ups. So don't think for a moment that I was a bloke that turned up at role disposals. The same was true for every other member of the boards in their own way.
Acknowledging the OP's question was between FJ, Multis, Helos. Excuse the diversion, please.