GameStation are selling the Official Xbox 360 Universal Media Remote for the Xbox 360 for £5.99 + £1.00 Delivery.
You can get cashback on this through the usual sites (e.g. 5% Quidco, making this just £6.64)
Next best price seem to be £14.98 from Amazon.
Note: GameStation is one of the companies PC World lists in it's Price Promise. I could not find the remote on their website, but if PC World do sell it in store, then you can get the remote from PC World for the same price minus an extra 10% of the difference.
The Xbox 360 Universal Media Remote was engineered to serve as your integrated control centre for the entire Xbox 360 experience. Enter a new world of digital entertainment with the touch of a single button. Play DVD movies and music, plus control your TV and your Media Center PC with one controller. The centralised, glowing Xbox Guide Button gives you quick access to your digital movies, music, and games library, and backlit buttons make the keypad easy to see in a darkened home theater.
Power your Xbox 360 system on or off without leaving the couch.
The Xbox Guide Button allows for easy navigation of the Xbox Gamer Guide.
The Xbox 360 Universal Media Remote includes control buttons for your music, pictures, and more.
The Xbox 360 supports progressive-scan for DVD playback functionality, including a control for opening and closing the tray.
The remote supports universal TV and console functionality, including volume and mute buttons and up-and-down channel changing.
Windows® XP Media Center Edition Button and Media Center Edition functionality are featured.
A, B, X, and Y buttons are included for basic control of your Xbox 360 system, even without a controller.
Numbered and backlight keypad gives you full functionality, even in the dark.