The UK has 6 Coenagrion species and the females can be very tricky to differentiate, so similar can they appear. Females of 2 of the species both exhibit 2 different colour/pattern forms so that there are actually 8 combinations to consider. In some cases there is just no substitute for the fine detail of the pronotum being visible.
Only 2 of the species are widespread and one of those is very patchy. the remaining 4 have very restricted distributions so location can help narrow the options.
There are 2 species, both having restricted distributions, which do have distinctive abdominal patterns, though.
Irish Damselfly (Coenagrion lunulatum) female
Not being in my own catalogue, this image of C. lunulatum is reproduced courtesy of Wikimedia Commons photo: Andreas Thomas Hein, CC BY 3.0, via Wikimedia Commons
Dainty Damselfly (Coenagrion scitulum) female
Limited to a handful of sites in Kent, mostly with restricted or no public access.
If you have one of these then you probably went specifically in search of it and know what you have.
Our remaining options exhibit one of two abdominal patterns, both having more than one possibility.
Coenagrion female – solid black abdomen
Coenagrion female – interrupted black abdomen