I picked up a new one from Tottenham Court Road recently, but they're few and far between. Might be worth ringing Ask or one of the big ones to see if they've got one sitting in their warehouse. Alternatively you could get an X-box 360 and buy the HD DVD drive for £25 (Amazon certainly had this...).
And there's always E-Bay, although there's a big demand for players there. 2nd hand ones go for full price!