There is a slim chance that you could have spotted a Broad Scarlet/Scarlet Darter (Crocothemis erythraea) in the UK. It is represented by only 20 or so records, though, so 99% of folks getting here will be looking at a Skimmer (Orthetrum) species.
To be safe, these two genera can be separated by the last antenodal cross (Ax) vein, complete or incomplete, if you can see that detail.
Last Ax vein incomplete, a half vein = Broad Scarlet/Scarlet Darter
Last Ax vein complete = Skimmer
If that detail is not visible, a mature male Broad Scarlet/Scarlet Darter (which might be the most likely to turn up if anything) is bright red all over, thorax and legs included, so is somewhat obvious. Females are much more cryptic, though.
So, switching into colour mode briefly:
Broad Scarlet/Scarlet Darter, mature male
Entire body NOT bright red = Skimmer