Was in Blockbusters this evening and they had the Xbox 360 HD-DVD Drives in brand new for £2.95. They were labelled up as pre-owned but were still sealed, having been transferred from new.
This is nationwide and in store, but wont last long at this price! Also not sure which stores have them but my local (Tile Hill, Coventry) had some!
To check if your nearest store has any, phone Blockbuster CS on 01895 236760.