Ok, here's my penny-worth input to the subject.
I used to log all RAF callsigns to/from Malta as a young lad, from 1971-1979. Transit aircraft, especially fast jets used the Mxxxx type callsign. The second letter was either A,B,C,S,T,O,P. Transport Command had its own system, so, VIP aircraft usually used ASCOT 1xxx, ASCOT 2xxx were the VC-10s, ASCOT 3xxx were the Belfasts, ASCOT 4xxxx was for Hercs and the latter also used ASCOT 5xxx when supporting a detachment. ASCOT 6xxx was for Argosies. The Andover was at the latter stages of its life in the RAF and if I remember correctly they used ASCOT 7xxx, but I could be wrong
The tankers, when supporting the fast jets used RAFAIR and so did their receivers, only that the tankers used RAFAIR 8xx and their receivers used 9xx. The Navy Canberras, as did the Hunters used to use NAVJETxx on transits to/from Malta.
The local two squadrons, ie 13 & 203 routinely used 3 letters followed by 2 numbers for local sorties.
Happy to learn more about the subject.