Anything darker than standard tinting is illegal.
On front windows perhaps, I've seen plenty of minivans 4x4's etc with very dark windows not getting stopped. 70% light transmission sounds about right and yet still if you asked most coppers they wouldn't know for sure.
On front windows and they have kit for testing it now.
Yup they have some natty little gizmos, but they seem to only pull the "boy racers", hence why I was suggesting that if you don't go nuts then you should be alright. At least that is the way the rozzers round here seem to operate
"VOSA has spent £13,000 providing light meters for roadside checks. If the meter detects between 45% and 65% of light getting through then you would be asked to remove the tint, failing to do so could result in a £2000 fine. If the meter says 30% to 45% the vehicle is deemed unfit for the road and a delayed prohibition is issued leaving 10 days to remove the tint. Less than 30% and the vehicle cannot be moved until the tint is removed.
However there have been cases of people being caught without the aid of a meter. The law says that an officer can issue an immediate prohibition if he deems the tint to be dark in his opinion. This however has to Obvious. This still seems like a grey area as the resulting proceeding would be depend on the amount of light that does get through and without a meter this would be inaccurate. "
Above taken from ukmotorists.com
So as stated I'd go for a light/ lightest smoke.
