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 a 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.
In Italy, Amatriciana is quite often cooked with pancetta cubes (smoked or not).
Here in the UK, if you can't find pancetta (easily found at Waitrose or Tesco), you can also substitute with lardons, bacon cubes or streaky bacon strips. The overall effect is very similar - however I suggest that you try guanciale if you can find it.