Surely there has to be more to this than just being ill.

Q237: How/why are firearms certificates refused/revoked?

A firearm certificate may be fully revoked by the Chief Officer of Police for the area in which the holder resides if he has reason to believe -

That the holder is of intemperate habits or unsound mind or is otherwise unfitted to be entrusted with such a firearm; or
That the holder can no longer be permitted to have the firearm or ammunition to which the certificate relates in his possession without danger to the public safety or to the peace; or
That the holder is prohibited by this Act from possessing a firearm to which section 1 of the Firearms Act applies; or
That the holder no longer has a good reason for having in his possession, or for purchasing or acquiring, the firearm or ammunition which he is authorised by virtue of the certificate to have in his possession or to purchase or acquire.

Yeah, I know, they are subjects, not citizens, no real rights, etc.