You are supposed to declare every modification to the car. This includes painted interior bits or a nice CD player etc. This is because as far as the insurance are concerned, they make your car more attractive to people wanting a steal it. Its not always down to how they think you will drive because you have a noisy exhaust system.
However, in the real world there is all sorts you can get away with.
Say you had an accident. Your car got written off and it wasnt your fault. Youve got undeclared mods and are ****ting yourself because the insurance want to send out an engineer to inspect the car.
Now, do you think it is practical to send an engineer from the Companies HQ in (lets say) London, to the policy holders house in Manchester?
No. Its not. Insurance companies hire independant companies from your local area to inspect THE DAMAGE. Not to inspect what mods you have on your car! The person inspecting your car doesnt know what mods you do and dont have. They have no information atall as to what your policy covers. All they know is that you have had an accident and they need to write a report stating the condition of the car so that the insurance company can declare it as a write off or not.
The only area this may become a problem is if a modification has been damaged. Say you have a massive great body kit that has been smashed to pieces. The engineer will inspect the car and make notes that the bumper has been damaged and will have to take photos. Now if the car IS a write off, it wont matter, the inspectors will tell the insurance co its a write off and they will write it off. But if it isnt, and the insurance co have to pay for the parts to be repalced, they wont be happy when they find out they arent replacing a standard bumper.
So what im trying to say is obviously stuff like bumpers and wheels need to be declared as they will get damaged in a crash. Stuff like brakes and performance mods neednt imo. Its a matter of using your common sence. If you crash your car, your insurance company arent going to come around and take your engine apart looking for uprated cams and larger throttle bodies etc. They will hire a third party company to come and look at your car and report WHAT IS DAMAGED, not what mods you have on your car.