I suggest you talk to Brian Samson who has gone through this process at a disused building in Doncaster.
Brian did need planning permission and he also needed to persuade the environmental officer about the noise from the building and the shooting activity regarding potential complaints.
Edit: you are right that the police have no official role to play if you keep sub 12, but it will pay in the long run to keep them informed. If you want grant assistance then the tacit support of the police will be valuable.