Staying inclusive and strictly formal requires a clear view of the fore-wing now, specifically of the grandly named post-discoidal area/field, which could be tricky to see clearly. By inclusive, I mean accounting for two much less-than-likely rare vagrants.
One, Pantala flavescens (Globe Skimmer/Wandering Glider), is extremely rare with only 4 accepted records. In the unlikely event that one did turn up, getting a decent picture would be even less likely. So, I’ve decided to exclude that. The second rarity is Crocothemis erythraea (Broad Scarlet/Scarlet Darter) with 30 or so records. For a lucky observer that one could be worth covering and climate change will likely increase the chance in the future.
However, 99.99% of those ending up here will be choosing between the likely UK resident genera [i.e. not rare vagrants] of, Orthetrum (Chaser) species and Sympetrum (Darter) species. Between these two the often more visible antenodal cross (Ax) veins will distinguish.
To account for the remote possibility of a Crocothemis erythraea (Broad Scarlet/Scarlet Darter), given sufficient detail the post-discoidal area/field lies outboard of the discoidal triangle in the fore-wing. Does this area narrow or expand/remain constant towards the wing tip?