admiral/elephant and the likes of tesco will give you a quote, but they add stupid amounts on for ANY modification.. and they dont cover the mod. Say you buy a set of wheels, declare them on your insurance and they get nicked... they'll only replace them with factory standard items.
3 that i'd recommend are Adrian Flux, Greenlight and Brentacre .. they ask a lot of questions but the prices are good and everything is then covered.