Yeah I think it's stuff like remap's etc that will effect your premium though generally a lot of mods tend to freak them out, any excuse to be A-holes.
Extra £110 for an exhaust system on Admiral to me. I've got two quotes, one with Admiral and one with Greenlight. Need to phone greenlight up to see if they'll bring it down a bit further or put a BHP limit on the car so further modifications are FOC.
Might try those who you said hoolio, do they have any restrictions IE Age, Location?