Game and Zavvi. It's also up for "pre-order notifications" on Amazon, the Microsoft Store, GameStop and Best Buy. Best Buy's ad lists the Xbox One with a "holiday 2013" release window, though all we know officially is that it's coming "later this year."
Game lists a pre-order holding price of £20 for the Xbox One, while Zavvi's pre-order down payment is £400. Zavvi also has a lineup of Xbox One games for £50, including Call of Duty: Ghosts and four EA Sports games.