There are two species of Semidalis in the British Isles:

Semidalis aleyrodiformis and Semidalis pseudouncinata

Both these species share the following features.

Diagnostic features of the genus Semidalis. RMCV (crossvein between veins R and M), MCCV (crossvein between veins M and Cu1). Image: Rintala and Lehto (CC BY-4.0)

To identify the individual species of the genus you need to examine the male genitalia, in particular the shape of the parameres (see image below).

Diagnostic features of Semidalis species. Images: Aspöck et al. 1980, Rintala and Lehto (CC BY-4.0), CBG Photography (CC BY-3.0)