I found this snippet but not much else.

Shotgun or Firearm? There is no reason why you should not be permitted to have one or more cannon if you already have a Shotgun Certificate (SGC) or Firearms Certificate (FAC). A smooth bore of less than 2” calibre with a barrel at least 24” long is a shotgun and can be placed on a SGC. Anything else is a firearm.

I'll have a mooch and see what I can find out.