To clarify, I think that ideally the 1500 hrs needs to be fast jet or mix of fast jet/rotary as a test pilot.
But previous fast jet experience is degraded with time, possibly. As previously mentioned, the L29 accident, was many years after the pilot had left the Red arrows. There is no easy answer to experience or currency.
I think the CAA are going to have to take a holistic approach to all this!
Finally as to SEP display aircraft, such as Spitfires, Hurricanes, etc., the CAA might want to adopt the very sensible rules set down by the RAF and BBMF in terms of currency.