Probably the cheapest price offline. Since the supermarkets have been forced to sell it at around £40. Also, for those that dont know there is a shortage of PS3 stock atm, so Argos might be your best bet for Friday morning hunting. At the time of posting this deal the check and reserve online still isnt taking reservations. However, the ring and reserve is live and ive just reserved my 360 at my local 10 minutes ago.
EDIT: It seems that you can tradein selected games at Argos nationwide to get it for either £22.49 or £9.99. Info link below in comment 15. Also seems to be cheapest price anywhere, including online. With only Amazon coming close at £35 for 360, but no direct stock of PS3.