What can I substitute for Guanciale? Guanciale is something I've never seen in the UK. What can I use as a substitute?

Or, alternatively, is there somewhere I can source this in the UK?

Note: I'm thinking of experimenting with an Amatriciana.
Wikipedia suggests Pancetta which is another form of cured Italian bacon, usually without smoking. I think the primary thing you are looking for is cured but not smoked, as smoke will add a strong flavor that is completely missing in Guanciale.