A small iridescent green beetle up to 5mm long. Picture taken of dead beetles brought home and then photographed.
No interactions present.
A Green Dock Beetle (Gastrophysa viridula); but you've picked probably the largest order of any type (the Coleoptera) in the UK - over 4,000 species, I think. So it's possible I'm completely wrong!
Lat/Lng: 51.8241, -2.733
OS grid ref: SO495141
The beetles were in a swarm of thousands walking all over the lane and in the adjacent hedgerows over an area of several yards (20?). None were flying. They were so thick on the tarmac that we had a job to walk without treading on them.