This is taken from a pdf file I found on the net from VOSA (Vehicle & Operators Services Agency)

"If you're selling a vehicle with extra tinting applied to the windscreen or front windows - The vehicle may now have glass that is darker than permitted by Construction & Use regulations, in which case the vehicle should not be sold"

And the limits are: "Motor vehicles first used after April 1985 - the windscreen must let through 75% of light and the side windows must allow 70% of light through"

All of this means Ford could not, in no way whatsoever, sell a single ST with privacy glass and yet they do. Its because it applied to the rear windows and not the fronts. Amen. *nods*

http://www.vosa.gov.uk/vosa/publications/tinted%20windows%20information.pdf

These stats come from a police force website:

Front Windscreen: 25% (they must let 75% light through)
Front Windows (i.e drivers and passengers): 30% (70% light must get through)
All other windows – No restriction.