IIRC, the Primary legislation refers only to the two Proof Houses (Gunmakers & Brmingham).
I would guess that Enfield would claim Crown Immunity (the Law of the land doesn't count) when selling to Police Forces. Whether or not they were correct in doing so, in my opinion, that would not be the case if a Police Force sold the rifles on. In other words those rifles would NOT be in Proof and anyone selling them would be guilty of an offence.
However if the Constabularies that did this also claim Crown Immunity (it would be tougher for them than for Enfield), then the dealers who sold them on could be guilty of an offence. As would anyone in the UK who currently holds one. If it has not be Proofed in London or Birmingham, it is not in proof.