Front side windows must be able to transfer not less than 75% of light. Even some of the clear security films can fall below this level. Have heard that you can incur a 3 point fixed penalty for tints but couldn't say for certain
From my understanding there is no specific offence which a ticket applies too, but there are two offences which can be used (I can't remember them off the top of my head). One is a £30 fixed penalty, the other is a £60 one with 3 points. Which you get depends on the officer involved and any aggravating factors.