The Resident Evil 5 demo was released December 5th on the Japanese Xbox Live Marketplace. Even though all dialogue and menus are in English, this demo isnt available for anyone outside of Japan.
Here is a way you can play it in the UK :-)
1. Download these five rar files from here: http://www.mediafire.com/?sharekey=fdf4f1773666296791b20cc0d07ba4d2e66f2dcbbe596359
2. Extract Resident Evil 5 Demo.part1.rar with the rest of the rar files in the same directory.
3. This should create a folder called content.
4. Create a data disc and burn the content folder to a CD or DVD.
5. After the burn is complete place the disc into your Xbox360 and the Xbox should read Mixed Media Disc.
6. Finally, go to your Game Library and the Resident Evil 5 demo should appear there. Enjoy!