These three recent releases are even cheaper than play.com currently. I know that you have to pay £2.45 in postage at bbc shop, but if you use the attached voucher that snag is eliminated.
They also have a number of other titles at £6.79, most of which make them the cheapest around online, but do double check before buying (a few titles are cheaper at lovefilm etc).
You might want to be wary of buying The Robots of Death, even at £6.79. There's plans afoot to reissue it (much like The Five Doctors was this year) with improved picture quality and extras.
Don't forget quidco.