Not sure about the callup status: I know that RAFVR(T) officers (i.e. ATC/CCF) are exempt, and I'd be surprised if RAFVR were subject to callup as they don't get adequate training for operational duties.
I'm interested in the question of logging/counting UAS/AEF* flying, though, as I'd been told several times that I couldn't log time flying with UAS/AEF instructors unless they were also CAA FI's, since I was neither P/UT nor P1.
Tim
*Air Experience Flights: usually the same aircraft/staff as the UAS, but called this when providing flights for ATC and CCF cadets.