I’m amazed no one has mentioned the CP1, single shot, bolt action, CO2 pistol. German design, made in China, to a pretty good standard actually. Magazine version available, but some reported issues. My single shot is more accurate than I am ever likely to be. Around 100 quid.

And if you want to move up from plinking to competiton at a not too serious level, then I second the recommendation for UBC. Lots of comps, lots of pistols, good bunch of guys. Great fun.