Ah sorry you are correct - he wasn't in the group, but he was one of the people who organized it, at least according to the marketing blurb:

"The Ministry Of Ungentlemanly Warfare – In theaters April 19. Starring Henry Cavill, Eiza González, Alan Ritchson, Alex Pettyfer, Hero Fiennes Tiffin, Babs Olusanmokun, Henrique Zaga, Til Schweiger, with Henry Golding, and Cary Elwes.
Based upon recently declassified files of the British War Department and inspired by true events, THE MINISTRY OF UNGENTLEMANLY WARFARE is an action-comedy that tells the story of the first-ever special forces organization formed during WWII by UK Prime Minister Winston Churchill and a small group of military officials including author Ian Fleming. The top-secret combat unit, composed of a motley crew of rogues and mavericks, goes on a daring mission against the Nazis using entirely unconventional and utterly “ungentlemanly” fighting techniques. Ultimately their audacious approach changed the course of the war and laid the foundation for the British SAS and modern Black Ops warfare."