I know a guy who works alongside the police sometimes and is pretty knowledgeable on this topic. The law has not changed, it is just a clarification that has occurred of existing law.
The light emission into a car throgh the glass I believe (without checking - Im sure theres a DVLA website with info somewhere) refers to the windows that the driver uses for his vision and this COULD also include rear windows if the police want to be particularly picky : yes I know a van has no rear side windows but thats a van not a car, and a car can be seen as different in the laws eyes.
The light transmission through side glass must be IIRC > 72%, so your tint must be no greater than 28%.
1 common problem is that you go to your tint shop and get, say a 15% tint applied. Youre fine therefore. NO YOURE NOT ! You have ADDED a tint of 15% to your existing glass which on some cars come from the factory with a tint of up to 25%. You could therefore now have a tint of 40% = illegal.
All these big F**k off X5s etc that have some silly bint driving it safe inthe knowledge that when they use their mobile whilst driving are safe from the eyes of the law have probably had their dealer say to them " ... and madam would like to have some nice tinted windows fitted ..." and she says Yeah, go on then. Polie stop the X5 and bint says "but they must be legal, this is how BMW make them", Bollx they have been added after manufacturer and she and the dealer are both liable to prosecution.
Many of the window tint companys have been acting illegally and they knew what was legal and what was not - they should be measuring the existing light transmission level and then refusing to apply a tint that would rended the vehice illegal. Many owners would have still wanted the tint applied knowing its ilegal but there is a good reason for the law : as a cyclist and motorcyclist its worrying to approach a car sitting at a junction with dark tints not having a clue if the driver is able to see you or not - even worse when it is night-time.
I have heard of one guy being stopped with dark tints whilst driving with his side windows down. He said that if it was dark then he drove around with the window down so that he could see out properly : so if its pi$$ing with rain and dark and as he doesnt want to get wet he keeps his windows up ..... he isnt able to see a damn thing - plonker, dont risk killing someone just because you want to keep one aspect of your car looking cool, it aint worth it.