The law has now changed. As before, you can have whatever tints you want on the back windscreen and rear side windows- essentially making your car into a van if they are dark enough. Front side windows were always a grey area as often a light smoke tint was still legal but sometimes on top of a factory tint they could come out slightly too dark.
To get round the problem that coppers couldnt tell which cars to pull and save all the mucking around with lightmeters, the new law is basically, ANY aftermarket tint on the front windows is illegal. The more reputable tinting firms are now offering a tint removal service for free if they had previously fitted a light smoke to the fronts under the previous law. There is nothing to say that you cannot drive around with them on as long as you want, as the law only recently changed, however, no NEW tints are legal on the front and any old ones spotted will apparently be given a rectification order - get it sorted in other words.