If you don't declare your mods, you're an idiot. End of.
Not declaring things like tinted windows and exhausts is the pinnacle of stupidity. You think an assessor won't check when they pop round to look at your car after you've ploughed into a Veyron or a bus stop full of kids and they're looking at ways of reclaiming their money? And don't even think about removing them if you crash, that's not going to work if you're in hospital with two broken legs is it?
If you don't know you have mods, then you ask. Common sense up to a point of course (bulbs, wiper blades, tyres no; air filter, exhaust, gear knob yes), but for fussake don't give insurance companies a reason not to pay out.
Why would you spend £400 a year and risk not being covered? Might as well not bother with insurance at all in that case...