Hi,
It's fair game as long as it's on YOUR land (Wildlife & Countryside Act 1981).
All birds and animals are protected by law. Certain species are classified as pests or vermin, and only these species can be legitimately shot. This can only be done by authorised persons. An authorised person is someone who has the proper permission from the land owner to control pests on that land.
Air Gun Prey.
The following pests are considered suitable for controlling using an air weapon.The UK law is 12ftlb for a rifle and 6ftlb for a pistol.
Brown Rat, Grey squirrel, Rabbit, Crow, Rook, Magpie, Jay, Wood pigeon, Collared dove, Feral pigeon, House mouse, Mink.
Not sure if the same rule applies to other vermin (MP's, Hawkers, Call Centre Workers, Tele-Sales, etc) they're probably protected by some daft EEC regulation (pity though).
Regards, Farmer