Definitely Ronnie Sunshines for airgun stuff as that's their main market. They also have an indoor range which I sometimes use during winter if I want to plink but not be freezing cold.

Park Street Guns is my local shop and I usually buy my cartridges there. No gunsmith though, so I'd speak to Ladbrook & Langton up the road from there for any gunsmithing requirements. Atkin Grant & Lang in Markyate has a gunsmith as well I think, and it's the best place I've found so far to shoot clays.