as chris says they all need to be declared to the insurance company or they could get arsey if you needed to claim. my company had a mod premium which basicially cost £25-50 which you paid after any modification (meaning your first mod not every mod you do) then the premium would go up depending on the mod you do. some dont have much of an affect others will shoot up tht total.
if you dont want to screw yourself over just ask them when you call them what their policy on modifications are and how much it would cost if you changed... i dont know lets say... alloys, exhaust and suspension. if they say oh we couldnt insure you with those alterations or that would cost $1million then you say oh i guess i wont do that then... no problems. if they say it would cost an extra £50-100 you say that you are going to do them in the next few days and could they add it to your policy so you are declared ready for the mods to be made... bobs your uncle fannys your aunt
i guess you just got to run the risk if you decide its too costly and you have already done the work
i tried them out! very friendly and helpful but the bloke even said stick with the churchill deal! 
