It isn't illegal to have them tinted but they need to comply with the below:
The front windscreen must let at least 75% of light through and the front side windows must let at least 70% of light through.
Sauce: Direct gov. I also work in insurance.
But for the sake of a slight tint thinking about it I don't know if it sis worth the extra 70 quid. Films that allow 80% light through are available but looking at a scale it don't really make much different I don't think.