I don't think the RAF ever had a dedicated cadre of permanent Instructors (except during the early part of the war, when we wanted large numbers of operational pilots quickly). Then they roped in all manner of too old for RAF service civilian instructors for full time duty (cf the poem "The Flying Instructor's Lament", which is on here somewhere).
But the general principle seems to have been that each generation of pilots had to teach the next. On the Arnold Scheme in the US, we sent over 7,000 + LACs ex ITW, got 4,000 + pilots back, and they kept 500 + "Creamies". Selection was at "Wings" stage. They were told that their function was to replace the American civilian Instructors who started up the six "British Flying Training Schools" which were opened (under RAF Command) in summer 1941 in parallel with the US Army Air Corps Arnold Schools.
Nevertheless, there was at least one RAF "creamie" (P/O MacMillan) at my Arnold Advanced School. The US Army used the same system: in their own Basic and Advanced Schools, all Instructors were "creamies" - and loud were the lamentations after Pearl Harbor when they saw their erstwhile schoolmates going off to fame, fortune, glory and promotion in the "Mighty 8th" while they were held back until they'd done 12 (?) month's Instructing.
Danny42C.