The UK Road Vehicles (Construction & Use) Regulations 1986 Act details the legality of window tinting in the UK - and as amended specifies the minimum levels of light that must pass through the windscreen and front side windows.
For a legal window tint in the UK the specifications are:
a) Motor Vehicles first used on or after 1 April 1985
b) The light transmitted through the windscreen must be at least 75%.
c) The front side windows (to either side of the drivers’ head) must allow at least 70% of light to be transmitted through them.
If the glass is tinted to a point whereby it lets through less light, the vehicle is breaking UK Window tinting law.
hope this helps