Sainsburys are running the following offer at the moment:
£60 off Xbox 360 consoles when you buy selected games and accessories.
They are selling the 250GB version (4GB version only available with Kinect and priced at £249.99) for £199.99. When adding 12 months xbox live to the shopping cart together with the console the price is showing as £159.99 which in fact is a £70 discount so even better value.
So to get this offer simply add the 250GB console and 12 months live to your shopping basket and the discount will be applied automatically. You can of course add buy something else from the selection but the 12 months live represents the best value.