We apparently had an ACP role. I don't know whether I was supposed to run to the squadron like my messmates but nobody ever complained. Anyway my stock excuse for being late, being a bachelor, was that I hadn't got back to the mess until 0730.
On one occasion our squadron commander was walking around with a black cloud above him. All our aircraft on the station were u/s except one and he was supposed to provide an ACP aircraft plus a spare. I suggested that he nominate a u/s aircraft as the prime and the serviceable one as the backup. i.e. Jump in, it's u/s; go to the backup.
A bulb flashed in his head and he zoomed off to his office.
Valiants never backed up fighters for QRA or anything like that.