Strictly speaking; as dark as you like behind the middle door jambs, complete "limo" black-out if you like. In front 0% beyond what is on your car when it comes out of the factory. I think there is an official figure of about 25%, I'm sure someone will come along with the correct information, but it depends on how silly you are and the mood of the plod as to whether they're going to bother with you.
I think if you were diving like an arse and then gave them lip when they stopped you they would make a point of it, but generally if you're driving within the law and they are not blacked out they seem to turn a blind eye.
I have driven past many police cars now and they have not said anything at all. There are many cars in Bedford with tints at least as dark. I think the location of the police force is a factor too to be honest.
The picture makes the tints look darker than they are; you can actually see into the car fine if you look carefully, it just seems to blur the features of the people inside rather than totally obscure. The best bit for me is how much cooler the interior stays when the car has been sitting in the sun all day.
When you're inside the car you can't really tell they're tinted.
Wait until winter time when it is dark and raining, you will then notice that your vision is impared due to the reflective properties that the film has, you will be looking at through the window and be distracted by the reflection of the interior lights/dash lights.
Vaux tint the windows to by 30% this is the legal max, if you have gone 25% tint then that means your glass currently is tinted by 55% thus meaning it is now only 45% clear as opposed to the legal 30%, if you were to be stopped you would be made to remove it there and then i believe, H21 would be able to confirm that.
the way the law is being tightened up i personally wouldn't bother, many more police forces are carrying equipment that can measure the amount of light coming through and they are now not only targeting the 'boy racer' element, you would be surprised the amout of luxury car owners and members of the 4x4 brigade who are being picked on