Rear Windows:
There is no legal requirement for the rear passenger windows and rear windscreen.
Front Windows:
As per the Road Vehicles (Construction and Use) Regulations 1986, Motor Vehicles first used on or after 1 April 1985 – the front side windows must allow 70% of light through, the front windscreen must allow 75%.
Failure to comply with these regulations can result in a prohibition notice, stopping you from using the vehicle until the tints are removed, a penalty notice or court summons, as well as the possibility of invalidating your insurance.