Since steam UK seem to have been blocked selling it direct I thought I would post this.
Shopto.net has the game for PC for £22.85 and the best bit is they give you a serial you can activate in steam :)
Why steam cant sell it direct is bizarre but this is cheaper then it would have been anyway.
Warhammer 40 000 Space Marine - Download - ShopTo.Net
*confirmed working as I got my copy from there*