Road Vehicles (Construction & Use) Regulations 1986 as amended specify the minimum levls of light that must pass through the windscreen and front side windows. The limits are:
Motor Vehicles first used before 1 April 1985
The windscreen and front side windows must allow at least 70% of light to be transmitted through them.
Motor Vehicles first used on or after 1 April 1985
The light transmitted through the windscreen must be at least 75%. The front side windows 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, then the vehicle does not meet legal requirements.