There are four species of snakefly found in the UK. They're difficult to spot and pretty under-recorded as they spend most of their adult lives in the canopy, only seen lower when they first emerge as adults in the spring and summer (after 2 years under bark as larvae) or to oviposit (lay their eggs).
This one is a female Xanthostigma xanthostigma. The sturdy pointed ovipositor shows that it's a female. Incidentally, this species is a good one to remember the name of if you're playing the alphabet game with a suitable category (there aren't too many options for X).
They're predatory, feeding on other insects, especially aphids. The larvae are also predatory, feeding on beetle larvae under the bark of deadwood.
The common name comes from the apparently snake-like appearance of their elongated prothorax. Other than this and the colour, they look very similar to lacewings. The German common name, Kamelhalsfliegen (camel neck fly), also suites them rather well.
The Snakefly is one of the more unusual species that makes an appearance in 'The Tree That Held The Moon' to inspire curiosity in the wonders of nature.