Here are the hard facts from someone who lives in the UK and shoots his own content:
Prior to May 2004 a girl of 16 could pose for nude photo's if her parents were present at the shoot and gave their written consent. It was illegal to pose for porn under 18.
Since May 2004 it is now illegal for under 18's to even pose topless or appear semi clothed, as in ligerie, provocatively and the parents can no loonger sign their consent..
Where the UK is a real arse is that this law is retroactive. This means that anyone found in posession of topless or nude images of models under 18, even if shot before May 2004, when it was legal to do this face legal action.
A model release is not a legal document. This is to show that the model consented to the shoot and gives over all claims to rights of the images from the shoot to the photographer.
My favourite model at the moment:
