A large (6-7mm)  leafhopper associated with Black Poplar and is hybrids.  The  overall colour is reddish-brown, with fairly plain wings.  The vertex and pronotum have fine transverse wrinkles and the forewings are along wrinkled along the veins - these features are unique to this species in this subfamily. 

Other Idiocerinae found on poplars

Good quality photographs from side, top down and face, showing wrinkled  vertex, pronotum and forewings 

Black poplar and hybrids, including Lombardy poplar

Local (UK Auchenorrhyncha checklist 2021)

Rarely recorded

https://www.britishbugs.org.uk/homoptera/Cicadellidae/Idiocerus_decimusquartus.html