What common calibres are in the UK?

[Steve]

.308 / 7.62 and .223 / 5.56 are very popular in the UK, because army surplus ammo is available for plinking and when loaded with expanding heads both calibres are deer-legal. .22LR is surely the most popular firearm calibre, and being so easy and cheap to shoot many people are allowed to keep hundreds of rounds.

Component numbers are not restricted, but a dealer is required to inform the authorities if someone seems to be buying an excessive amount - it's all a bit vague - components are also stupidly expensive here.

Given the limited size of game in most areas I think a 22LR and 12g would be most useful, improvised slug 12g would deal with the occasional deer / cow / bull.