ACP of course had two meanings. Only the Variant was designated with a Command function. However unlike the SAC Looking Glass which had a one or two star general on board the Variant would only have had a standard crew. Its ACP role was probably defined in the Bomber Command War SOP (1st Edn). Coincident with the grounding of the Valiant the SOP 2nd Edition was published in 1965 (that is another story).
As far as fit and practice I don't know (also another story).
I will ask a researcher I am helping.