Lanchester submachine gun
The Lanchester is a submachine gun ("machine carbine") assembled by the Sterling Armament Company, W. W. Greener and Boss & Co between 1941 and 1945. It is an evolution of the MP28/II and was manufactured in three versions, the MkI, MkI* and the MkI**.
Source: Wikipedia — Lanchester submachine gun (CC BY-SA 4.0)