I published a post on how much the Xbox One will cost and Amazon had it on sale at £599.99. But if you're looking for a cheaper price then you definitely want to check out Zavvi.
Zavvi currently have the Xbox One up for pre-order at £399.99, a full £200 cheaper than Amazon. They also guarantee to charge you with the lowest price if the price does decrease when it comes to release date.
£399.99 for the console is still a bit steep because the original Xbox 360 cost around £300. But then this Xbox does have a lot more power and a lot more features.