Another thought would be lifting the E Branch background from Brian Lumley's Necroscope series of novels and recasting it in the 40s.
On uniforms you could pick up on the early LRDG, SAS and Popski's Private Army. A mix of service and non-regulation with individual wearing their original unit badges or you could put it on more formal lines.
Also SOE operated completely in civilian kit and were integrated into the local populace.
So lots of options really...