This may not be considered such a good deal as the current £171 incl. Quidco deal posted on the forum at the moment, but may be useful to some looking to trade in.
Not sure of other models but Blockbusters currently offering £75 in-store credit for an original xbox 360 premium (non HDMI) 20gb model. This makes a new 360 in store £199.99 - £75 = £124.99
They're also giving 1 month free games rental when you collect your shiny new 360.