http://www.bbfc.co.uk/search/?searchwhere=db&q=skyrim is 15, PEGI rating is 18.
I've been trying to find out if the PEGI rating is advisory or legally enforceable in the UK, and different sources give different accounts. As far as I can tell, in 2009 the British government declared that the PEGI rating would become legally enforceable, but that it hasn't actually done this yet. So I
think that it's illegal to sell Skyrim to anyone under 15, and shops
might refuse to sell it to anyone under 18.
Good luck finding a clearer answer

.
[edit]
Huh. PEGI's website doesn't have a classification up for it yet.