Try bigger branches of places like Tesco and other large supermarkets. I know Tesco Extra stores stock lots of garden/outdoor stuff – they may well have tents!
Edit: http://direct.tesco.com/q/N.1999275.aspx Just checked on the Tesco website and it looks like they stock them. It’s from “Tesco Direct” – think a lot of that stuff is available from the really big stores, so if you have one near you, it shouldn’t be a problem.