EDIT: IGNORE THIS, IT NO LONGER WORKS. The items have dissapeared from the marketplaces all together (unless you live in the US) making the method unworkable.
Pretty simple really:
1. Set up an Xbox Live SIlver account, but state it is situated in the US.
2. Go into the RBN store with your US account.
The song should have a price on it while you are logged into your US account, but it will become 0 again if you go back to the UK account. So:
3. Download the demo of the song you want through the US account.
4. Go to the UK account. You should be able to access the demo song in Quicklay.
5. Play through the demo, then buy the song at the end.
(This was tested by 2 friends of mine that live in the UK. I currently live in the USA, and cannot confirm for myself.)