You can access it in the UK if you have a US iTunes account.
However I tried it out recently and was disappointed. I think I probably misunderstood how it works but it's not like Spotify - you can't seem to choose exactly what you want to listen to and the selection of music seems quite limited too.
I wonder if they're not pursuing the idea of international iTunes Radio because they're planning on introducing a streaming service instead from their acquisition of Beats.