The rules for the windscreen states that at least 75% of light must get through. For driver and front passenger windows - 70%.
There are no rules for the back passenger or rear window, which can be as dark as you like.
If your windscreen or front side windows are too dark, you could be issued with a prohibition notice, forbidding you to drive the car until the tints have been removed, or you could receive a penalty notice or be taken to court.
Tinted windows are not currently included in the MOT test.