The legal requirement is for the windscreen to allow 75% of available light through and the front side windows to allow 70% through. As all modern cars are fitted with tinted glass as standard adding any additional film to the glass will probably make them illegal.
The point about light meters being carried by the police depends on the area you are in. I am a VOSA examiner in central Scotland where most of the road policing departments have Tintman available to them. VOSA examiners also have Tintman and can prohibit the use of any vehicle that does not meet the minimum requirement.
The rear side and back windows can be tinted as dark as you want, as a previous answer pointed out, vans have metal sides and back doors and they are perfectly legal.
Tinted windows are not included in the MOT unless they are dark enough to, in the testers opinion, affect the drivers view of the road.
If you have your windows tinted you also leave yourself open to prosecution as we routinely caution drivers when they are issued with the prohibition notice, if they fail to comply they are reported for prosecution. That is potentially 3 points per window plus a fine.
My advice is to spend your money on something to improve your car, not to make it more dangerous.