One of the best I've found was Cleveland Models, it was run by a guy called Simon I think. (Not been there in about 5 years.)
He had a couple of boxes of most kits, a good supply of spares, good access to hop ups and was really good for advice too
I remember being in there when someone brought in a cheap Chinese nitro and asked him to fix it, he took a good look, advised them well and sent them away happy with some good sound advice. Not many around who would do that.