What were some Secrete Intelligence Service operations in WWII? I am aware of the SOE and their insurgency, reconnaissance and demolition/sabotage operations (such as the operations in France, the assassination of Reynard Heydrich and the raid on the Norwegian heavy water plants), and the work of the Special Liaison Unit disseminating Ultra intercepts, but what other SIS operations were there?
I suggest checking out the recent official history of SIS by Keith Jeffery; unlike Christopher Andrew's official history of the Security Service, MI6 insisted on strictly limiting the scope of Jeffery's inquiry to its early history (the period 1909-49.)
But while that means it's not a great source of insight into British foreign intelligence during, say, the Cold War, it does cover the Second World War in detail. I'm on my phone right now, so sorry I can't give a more fulsome answer, but the general story is that SIS got squeezed by its sibling agencies (MI5, GC&CS and especially the SOE), which all grew significantly in terms of resources, political capital and prestige. Consequently there was a lot of turf-warring within British intelligence.
SIS remained the premier agency in the human intelligence field, and played an important role as a coordinator of and clearinghouse for intelligence, but it achieved fewer spectacular successes than its sister services, especially in the earlier years of the war — and, conversely, experienced some of the more spectacular failures (case in point: the Venlo incident.)
This is a very broad answer, I know — I'll try and swing back around later with some less general comments!
edit: typo