Technically, this a comparison chart for the GB Bluets. For it to be a chart of the UK Bluets it would need to include the so-called Irish Damselfly (Coenagrion lunulatum) which is sadly missing from my personal catalogue.
My chart covers only the males, which are primarily distinguished by the black design carried on the dorsal side of S2. Since this design exhibits variation though, some care is required.
[Distinguishing the females is particularly onerous since the visible markings are so painfully similar between species; good macro shots of their pronotums (pronota?) are required.]
Click on this diagram if you need to open a larger version.