There is a very slim chance that you could have spotted Crocothemis erythraea (Broad Scarlet/Scarlet Darter) in the UK. As it it represented by <30 records, 99.99% of folks getting here will be looking at an Orthetrum (Skimmer) species.
These two genera can be separated by the last antenodal cross (Ax) vein, complete or incomplete, if you can see that detail.
A mature male C. erythraea is bright red all over so glaringly obvious. Females are much more cryptic, though.