factory tint, from what I red on car forums and dealer technical sites is around 8%. I do know some police coprs have a device for testing the tint on the front, if they want to be harsh, they will use it and you may have to rip the tint paper off the windows. In the back you could basically paint your and it would not matter.
Of course, technically any modifications done to a vehicule is illegal, but imagine if they would enforce it how a complete market branch would be crashed down, from tuners to racers and ricers too. Also, I find it funny that it is technically illegal to swap your front, one piston calipers and rear drums with a set of 4-6 pistons brembo and disks all around. I also think its funny that a Reliant Robin is perfectly street legal, well in the UK at least. lol