the letter of the law. hope this helps.
Light Transmission of Glazing - Summary of requirements (Summaries of construction and use regulations for motor vehicles – Department for Tansport )
Motor vehicles first used between 1/6/1978 and 31/3/1985 must have windows which allow at least 70% of light to pass through. Motor vehicles first used on or after 1/4/1985 must have windscreens which allow at least 75% of light to pass through - all other windows must admit at least 70% of light.
Where such a vehicle is stopped and the window tints applied are such that the Visible Light Transmission level, when measured using an approved device falls to below prescribed levels, the following enforcement guidelines have been agreed with, and recommended, by the Government.
Above 30% Visible Light Transmission (Less Severe Window Tints)
The owner or driver of such a vehicle will be required to have the tinted film removed from the windows under the direction of either a Rectification Notice or a Delayed Prohibition Notice, A period of grace will apply for a limited number of days (normally ten) during which time the vehicle may be driven whilst the rectification work is to be completed. In either case, the vehicle will need to be inspected by either a Police Officer or Vehicle Inspectorate Officer to confirm that the glass has been restored to a compliant condition. Prosecution is unlikely in such circumstances provided the vehicle owner complies fully.
Below 30% Visible Light Transmission (Excessively dark Window Tints)
The owner or driver of such a vehicle may be issued with an Immediate Prohibition Notice and immediately prevented from driving the vehicle on public roads until the tints have been removed and either a Police Officer or Vehicle Inspectorate Officer confirms that the glass has been restored to a compliant condition. It is also possible, depending on the severity of the offence, that the owner may be prosecuted for driving a vehicle in a non-roadworthy or even a dangerous condition with the potential for Penalty Points and a Fine.
Driving such a vehicle on public roads before the tints have been removed and before a Prohibition Notice has been lifted will be a serious offence and the owner or driver is likely to be prosecuted.