The following extract is taken from https://www.gov.uk/tinted-vehicle-window-rules
It’s illegal to fit or sell glass (or a vehicle already fitted with glass) that breaks the rules on tinted windows.
The police or the Driver and Vehicle Standards Agency (DVSA) vehicle examiners use light measuring equipment to measure window tint.
If your windscreen or front side windows are tinted too much you could get:
Whilst we are aware that some companies will tint front windows, we WON’T. It will make your vehicle unroadworthy and would be irresponsible of us to condone this.