Firstly, PCPs are within your budget & easier to shoot accurately and to longer ranges then springers, CO2 are garbage.
Multi shot pcps are useful for pestcontrol as they avoid fumbling for pellets, especially useful in the dark if you want to do some lamping.
Your budget of £450 accomodates some good air rifles:
Air Arms s400 (single shot, the multi shot AA s410 will be nearer £500)
BSA Scorpion T10 (multi shot)
FX Typhoon T12 (multi shot)
The Webley Raider 10 shot now has a RRP of £299, seems to have a good rep BUT I don't have any experience of it so won't recommend it myself.