You have to have a section (1) firarms certificate to posess a cartridge, even a shotgun cartridge having less than 9 projectiles.
Thus if you were in posession of an 8 bore shotgun and a section (1) cartridge for that shotgun, it would be deemed to be a section (1) firearm.

You can get 8 bore shot cartridges in England but it will always be a search and a bit hit and miss.

For a normal certificate holder one can only shoot any particular caliber of rifle on a piece of land if that land is deemed suitible for that caliber by the chief constable of the area.
More experienced hunters may decide if that piece of land is suitible for the caliber without reference to the chief constable. Called an open licence, these are not given out like sweeties.

I hold a dealers licence and may fire or test anything anywhere I deem suitible. So I can test a game rifle by shooting game, irrispective of caliber, subject to the deer act minimum limits.

My 450#2 happens to be my personal gun and held on my privite licence. It is not a caliber that will be given permission of any chief constable for deer, thus it is a target rifle. However I can test it on deer as a dealer.

Regards