I do not know what the cause of the RAF Tutors accident was, but there is a possibility they were formation flying so PCAS would have been irrelevant.
If you take the recent AAIB report on the Staffordshire mid air (See separate thread) then you get an interesting picture. The vintage aircraft spotted the Micro, took avoiding action and was collected by a turboprop. The vintage machine and the micro had no transponder and there was no radar service available in the area. If the turboprop had had PCAS it would have failed to see either of the other aircraft, but see and avoid avoided the potential conflict between the vintage machine and the micro.
I fly from the same strip and the vintage machine and about 80% of locally based flying machines are not transponder equipped. It if far more likely that I will fit FLARM than PCAS.
Rod1