Why the big secrecy?
So much of the show revolves around the fact that these women are not allowed to tell anyone what it was they did during the war.
My father was in Canadian Army Intelligence during WW I. He always told us that he had had a job in intelligence, and that he couldn't say anything more about it; and we didn't press him on it. A few decades later, the official wartime history of his outfit (which much later grew into Canada's equivalent of the US NSA) was unclassified, and we learned a bit more.
It could not have been a secret that he attended Japanese-language school for a year or so in Vancouver (my own first year, as it happens); it wasn't a secret that he was then posted to Ottawa. It certainly hadn't been a secret, since Roman times or before, that cryptography and code-breaking was an important part of war.
Canada's Official Secrets Act, I believe, was modelled on the British one. I don't understand why our heroines, when asked "What did you do in the war?," just have replied something like, "I worked in intelligence," or even "I was a clerk," "and I can't talk about it." I expect that in just-post-war Britain that would have been a very common situation. Less dramatic, to be sure...