There is only one species of Megalomus in the British Isles, Megalomus hirtus, this species is easy to identify based on the features shown in the below images. M. hirtus is restricted in its distribution being found only in Scotland around Edinburgh and Aberdeen, it is associated with the plant Wood Sage.

Defining features of Megalomus hirtus. Photograph T. Rintala and T. Lehto (CC BY 4.0)
Megalomus hirtus defining features. Photograph: © S. Burgess