A narrow oval green bug found on Wych Elm and other tree species, including lime.  5-6mm long.  The basal (1st) antennal segment is more or less equal in length to the head, and has more than 5 (usually 7) long hairs.  The underside of the front femur has long hairs, and the 2nd antennal segments have several erect hairs. 

Other species of Orthotylus are very similar

Check for long 1st antennal segment with > 5 long hairs on it.  

Note the plant species on which you found it.  This cannot be verified from field photos; information about size and detailed photos showing head and 1st anntennal segment with hairs, plus front femora, are needed

Elm or lime

Widely scattered (ref. British Heteroptera checklist (2021) https://www.britishbugs.org.uk/systematic_het.html

Not known, but may be under-recorded.

Southwood, T.R.E. & Leston, D. (1959) Land and Water Bugs of the British Isles. Warne 
https://www.flickr.com/photos/tristanba/4792625989/ and https://www.flickr.com/photos/tristanba/4792625877