I would suspect that the cat P system is being used to raise the general alarm, so it is being used to protect the relevant persons, so it would be enforceable. If, as jokar says, the shout/gong is still in use, staff are trained in these procedures, and it is detailed in the risk assessment, then it would be difficult to enforce. But remember to appeal the notice within the 21 days allowed if this is the case.
However, if the cat P system has call points which I rather suspect it will have, then would it not be considered that this system could be better than voice/gong and as such it should be utilised in accordance with reducing risks ALARP.
If I inspected the place and found they were not maintaining a propertly protection system then I would be forwarding this information to our unwanted fire signals team, they may decide not to automatically mobilise to someone running such a system unless a phone call is received confirming a fire. (Kinda negating the entire system really)