Edward ... Acting is surely just a case of "Has made the list but all substantive posts have been filled". Thus a vacancy comes up, and away you go [as Acting] ... provided, as it used to be, that you meet the Job Spec for the appointment.
I've been there! Didn't get a post as Acting wg cdr, even though probably the best qualified guy in the Branch, as I was blocked by AOC [due to serving spouse, basically, as an independent Unit Cdr]. So a mate got it [as Acting wg cdr]. A couple of years later I was Substantive, as was able to fill an appointment where the only bit of the Job Spec was my rank and the fact that I was alive
Nigel and I went Substantive on the same List