Identification of UK insect
That looks like the cast off outer layer of a dragon fly nymph§:


The following resources may be helpful to get a more precise identification:
https://www.fba.org.uk/sites/default/files/EA_specid/ea_specid_mod_8.pdf
https://british-dragonflies.org.uk/odonata/species-and-identification/



§Note: As pointed out by Arthur J. Frost this is actually the exoskeleton left behind when a dragonfly nymph molted and became an adult.