From what I am aware, the BBFC is the main legal system, so I believe it is against the law for shops to sell games to those underage, but BBFC ratings are only really used on certain games that would not fall under the standard PEGI/ELSPA classifications, which are voluntary by the developer/publisher, and are realy just guidelines, and I'd guess is down to retailers policies on who they sell them to and if they want to follow those, but I'm not really sure on the full legal system.