I get asked the same questions all the time, mainly, what's a "legal tint for the front side windows?" well, to put it simply, there isn't any! The law states that the windscreen and both front side windows must allow 70% light transmission. How they got to this figure I'm not sure, but if you drive a modern Renault, Citroën, Ford, VW, Rover, etc, chances are that your front glass measures 65-68% straight from the showroom! Technically, the dealers broke the law when they sold the car! , what it means is that any film applied to front side windows will not be legal-ish. I say legal-ish because although the police state 70%, the ministry of transport should only issue an advisory if the glass reads 45% or lighter, but then if you're stopped by the police for tints (usually outside a max power show) it doesn't matter if you've got tints above 45% or not, the law is 70%. This applies to every vehicle on the road, no matter how old. What is more important is the "dangerous vehicle" level.
Any window film above 30% should only be ordered to be removed and maybe a statutory £30 fine. Any filmed window that measures below 30% is classed as "a dangerous vehicle" and can be instantly impounded and transported away, which you'll have to pay for, leaving you with a bus ride home with all your belongings! you could also be prosecuted for driving said "dangerous vehicle" which comes with a hefty fine and min 3 points-something to bear in mind if you're a new driver or have a bagful already remember, points don't mean prizes when it comes to keeping your licence. More importantly, your insurance could be invalidated should you need to make a claim.
Just to confuse you a bit more, the percentage rating on the film is not the same as the reading you will get when it's fitted to the glass. It depends on the thickness of the glass, the original tint of the glass and the quality of the film. typically,(with the film I use) 50% film will read from 37-32% on most modern cars 35% film will read from 23-18% on most modern cars 20% film will read from 11-7% on most modern cars 5% limo just manages 0.3% on anything.........its dark! So basically, nothing is legal but 50% should only get you a ticking off. I see a few ads on eBay saying tinting is easy, why pay a tint shop when you can do it yourself? Well, anyone can buy a couple of litres of car paint and spray their own motor-but would you want to drive it to a cruise and show it off after?