Legality G7
Regulations for automotive use
Automobile window tinting, by its nature, reduces the visible light transmittance through a car's windows. This can be problematic at night, when motorists must be able to see through the windows of other vehicles in order to spot hazards which would otherwise be obstructed. Police officers must also be able to identify the passengers in a vehicle.
In many jurisdictions, there are government rules and regulations in place to ensure darkness levels of films do not present a danger to motorists.
In the UK, the Department of Transportation specifies a minimum of 70% visible light transmission (VLT) for window tinting on the windshield and the windows to the immediate left and right of the driver. The DoT does not specify any VLT requirements for any other windows. The DoT regulates commercial vehicles, not private vehicles.
You can have a 30% tint on the front side windows. However, the police are advised to only caution the driver if a 45-50% tint is found on them (this comes from a police issued guidelines pamphlet).
You can have whatever tint you like on the rear window and rear side windows. You can even paint them matt black.
You must have NO added tint on the front windscreen, however.
Bear in mind that most newer cars already have a slight 5-10% tint built into the window glass themselves.