The law doesnt goven how much of a tint you can have, it governs how much light can pass through the windscreen and windows in front of the B pillar. Anything behind the B pillar you can have as dark as you like, limo tints are perfectly acceptable and there are no laws governining it at all. The fronts are a different matter.
Essentially your windscreen must let 75% of light through, and your front side windows must allow 70% of light through. This is where the problem comes in though, you cant just slap a 20% tint on the fronts and assume all is well. If I remember correctly even perfectly clear glass only lets around 90% of light through, pretty much every car will also have some form of tint on the windows. It's pretty much safe to assume that out of the factory your windows will only be letting 80% of light through. With this in mind even a 10% tint will be getting you increadibly close or possibly beyond the legal limit.
It's also worth noting that the punshiment should you get caught has more than one outcome as well. They are categorised into 2 categoris, less severe tints and more severe tints. If your reading comes in the below 70% but above 45% you fall into the less severe category and its simply a fine and 14 days to remove the tints / show evidence at your local nick that you have done.
If you decide to be epically daft and limo tint the fronts and fall below 45% you fall into the more severe category and your car can be taken off the road there and then.
It's also worth noting that your tints need to be declared to your insurance company.
Hope this helps a bit. One example of a source for this linked below.
http://www.shropshiretints.co.uk/legal.htm
