No, you don't have to to refuse the sale. Proxy sale law doesn't apply to games, DVDs, etc. If a parent wants to buy a PEGI 18 game for their 12 year-old that's entirely up to them. If the retailer wants to set its own policy on the issue then that's up to them but there's no requirement to do so.
Incorrect! PEGI ratings are enforceable under UK law and selling to a minor is an offence!
