This has been posted quite a few times before - https://www.gov.uk/tinted-vehicle-window-rules
99.9% of vehicles meet this criteria on leaving the factory.......ergo, you are not allowed to tint the fronts ('cos its already done)
whether you decide to risk it or not is your choice, many do and have done so for a long time without any issue, but...............if you have an accident that "could" be related to vision..........apart from any legal issue, what about your insurance ? - would they pay out.......I doubt it
Vehicles first used on 1 April 1985 or later
The front windscreen must let at least 75% of light through and the front side windows must let at least 70% of light through.