Confidence: It's likely to be this, but I can't be certain.
Notes: This doesn't seem to have the granular thallus of a Candelariella, and C. aurella scarcely forms a thallus at all. It would be good to test it with potassium hydroxide to confirm the purple reaction of a Caloplaca, but I am fairly sure it is one. The species is much less certain. It is not one I recognise, though a key to the group I think it might be in suggests a species that has only recently been recognised in Britain. However, I am getting into the realm of uninformed speculation if not fantasy. It would be really interesting to know the spore characters.