[robchalmers]

British insurers can be incredibly picky over the spec of your car. Quite a few will refuse to pay out if your car has mods you didn't notify them of - some will go as far as invalidating you policy leaving you in the legal nightmare of driving without insurance
Friends of mine have fallen foul of this previously. An ex-girlfriend was was hit by and another driver (who fled) and when the insurance guy came to see the car (pug 206) he spotted she had a set of aftermarket tail lights (nicknamed lexus lights). He promptly said that she'd modified the car with out declaring it and they weren't going to pay out.

when/If you mention you have a mod on your car in the insurers initial questions, they'll transfer your call to their modified car/young driver/high risk affiliate and charge you a hell of a lot more just because you want to change your wheels, drop your car an inch or put an noisy tailpipe on or any thing that takes it away from 'manufacturers specification' doesn't even seem to matter if you're doing it for ECO reasons either from what I've gathered