Tinting the front windows (ie forward of the B Pillar) is ILLEGAL. The reason is that you have to have 70% light transmission through the side windows and 75% through the windscreen. All car manufacturers tint windows in the factory that is why car windows have a blue or a green tint. This normaly comes in at 83% light allowed through. The lightest tint that i know is available is 70%, but when the window is already tinted 17% this will take it to 53% light going through. (illegal).
The penalty is a warning and removal at the roadside, a £30 fixed penalty and removal and a £60 FP and 3 points + removal depending on the darkness.
Just for your info, what you do with it is up to you. Tinting companies will always say oh the limit is 70 and i have a tint here that is 70. The reason? It is not illegal to tint the windows, just to drive on a road with them.