Not sure if u will get away with this one but must be worth a try

From another forum-

Tesco have recently changed the prices of the Core Systems from 184.97 to 197.97 and the Premium Systems from 259.97 to 279.97.

Some stores havent changed the shelf edge labels yet so still advertise at the wrong price. So if the core systems are advertised at 184.97 or the premium is advertised at 259.97 then you can get it for free. (Theres a special edition pack that comes with Pro Evo 6, i don't know if these are scanning correctly so make sure to ONLY GET THE STAND ALONE CONSOLE)

Buy it and pay for it, then go and collect it. Once you actually have the xbox, go to customer services and tell them they've over charged you. They'll more than likely offer to refund you the difference. Point out their policy stating "if they overcharge you they'll give you a full refund and you can keep the product" which is on the massive blue poster behind the customer services desk and also on every till in the store.

They should give in and give you a refund. If they don't tell them you'll phone head office customer services and then leave the store. Call HQ customer services and tell them that they refused to honour their policy. HQ will call that store, give them a slap on the wrist and tell them to refund you. Then all you have to do is go back and get a refund.

Sounds a lot harder than it, but seeing as you're getting between 200 and 300's worth of stuff for free it should be worth it!