The road vehicles (construction & use) regulations (as amended) lay down requirements which govern the amount of light that must pass through the glass of a vehicle (known as light transmission)

Regulation 32 states that the windows of any vehicle first used before 1st 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 1st 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.

WINDOWS WHICH THE LAW APPLIES TO
The windscreen and front side windows to either side of the drivers head.

HOW DOES EXCESSIVELY TINTED GLASS EFFECT ROAD SAFETY
Excessively tinted glass restricts the drivers’ vision, especially in dark conditions.  This may prevent drivers from seeing other road users and pedestrians.  It also prevents other road users and pedestrians from confirming through eye contact that they have been seen.