Hi, depends on what you want to use it for, If it's for vermin then I can't really help as I have no experience with killing vermin and not sure on what you can actually legally kill, I'm sure some one will be along to advise in this area.
If it's for target shooting then basically anything is fun, you can get fun pistols that replicate the real thing in almost every detail, for example you can get Beretta's, 1911's, Sigs and they all cycle and look just like the real thing, they have working slides, drop out magazines and even recoil due to the slide moving back and forth while some revolvers have swing out cylinders and even use brass shells, these are great if you're into replicas and a bit of tin can shooting in the garden.
If you want something with a little more accuracy pistol wise then I'd go for a pellet shooter with a rifled barrel, for example the Webley Alecto or a Weihrauch HW45, both these guns are excellent, powerful with good accuracy and are at the legal power limit. Here's a couple of vids..
HW45
Webley Alecto
Here's a couple of replica vids..
If it's rifles you want then with your budget a Air Arms TX200 Mk3 is certainly within your reach, this being one of the best spring air rifles you can buy although the rifle is just under £400 you may have to go over budget a little if you want a scope, actually you will need a scope as this rifle has no open sights. Here's a review of the rifle..
Check out some rifles and pistols here..
http://www.solware.co.uk/air-pistol-air-rifle/air-rifle-pellet-gun.shtml
Don't forget to check the law on Air guns with do's and don'ts. Hope this has been a little helpful for you and I'm sure someone will be along to give further advise who are more experienced than me.
Finally Bristol Gun club, will be able to give you all the advise you need.
http://www.bdrpc.org.uk/
Thanks a lot, I'll take a look at those links.