There are a couple of points here
1. the law states 70% - end of
2. as Ekona said so long as you declare them..................but because they are illegal - they will not accept it (or should not)
so even if you get your insurance to cover you, they could still attempt to wriggle out of any claim by the fact that they would exceed the legal limit
that said, I would still like to do it..............but won't 'cos couldn't afford to lose my insurance cover
also agree with cmp999 - a bit of film will NOT impede access to the car by the Brigade - they love playing with their toys and any excuse to make your car into a convertible