If you want an air rifle for plinking, then really any break barrel/underlever will do. If you intend doing something like Hunter Field Target, then a PCP (pre charged pneumatic) is the ticket. Need an air pump or a compressed air bottle like scuba divers use to refill the air chamber though.
PCP has no recoil, break barrel/underlever does.
.177 is generally preferred, as it is 'more accurate' due to a flatter trajectory, as already said.
If you are after a purely budget break barrel, you'll do worse than buying an SMK (Sport Marketing) rifle. Cheap and easy to use. If you are feeling flush, then a Weihrauch is a good bet. As Weihrauch have already been mentioned, I'll say this. IF YOU ARE BUYING A WEIHRAUCH, YOU
MUST ENSURE IT IS IMPORTED BY 'HULL CARTRIDGE' (as Weihrauch is German). HULL CARTRIDGE ARE THE ONLY IMPORTER OF UK SPEC GUNS AFAIK, AND GUNS FROM ANY OTHER SOURCE MAY NOT COMPLY WITH UK REGS IN TERMS OF POWER ETC. If it is from Hull Cartridge, then there'll be a sticker saying so.
Air Arms (
www.air-arms.co.uk) are a good gun maker as well, particularly their 'budget' PCP rifles. Your best bet Koa would be to buy a couple of magazines such as 'Air Gunner', and get some prices etc.
With regards to the law...............
You must be over 17 to buy an air rifle, but you can shoot one at 14 as long as there is a responsible adult present. Power must not exceed 12ft/lb. If you shoot in your garden, then steps must be taken to eliminate richochets, and no pellet can stray past your boundaries. If you shoot on land other than your own, you must obtain the landowner's permission.
Air rifles do not require a FAC (Fire Arms Certificate). However, guns that use air cartridges are now illegal, but AFAIK Guns using CO2 capsules are legal.