Adrian Flux seems very popular but I found their customer service terrible,. Been with Sky (brokers) for 4 years now for four year and even if slightly more expensive than Adrian Flux I have no complaints. Most "standard" insurers" wont cover power increase mods.
definite thumbs up for Greenlight - think they're even on this forum as an insurer provider. definite great service and didn't charge me for adding mods etc to the car, just ring up and job done. Theyre based in Norfolk too so you actually get through to a person and they're human, you can have a laugh n joke with 'em. Well recommended.