Your suspect is one of the UK’s three Emerald Dragonfly species. All have quite limited distribution with the Downy Emerald (Cordulia aenea) being the most widespread.
The Brilliant Emerald (Somatochlora metallica) has disjoint populations at the two extremes of southeast England and western Scotland.
The Downy Emerald (Cordulia aenea) has its core population in central southern and southeast England with isolated relict populations in Wales and western England.
The Northern Emerald (Somatochlora arctica), is restricted to Scotland, mainly western and above the Glasgow-Edinburgh line, with some presence in southwest Ireland. They are a dark black-green in appearance.
This can be done in monochrome but, other than eye colour, I don’t think colour varies greatly with age so switching to a colour comparison is safe and may make more sense.
For this photo of a splendid, immature male Northern Emerald (Somatochlora arctica) I am indebted to Pete Rose, with many thanks.
Mature specimens are even darker.