My impression was, when I used to research at the then Public Records Office, Kew, was that most items are released after thirty years, but some - in my case German prisoner of war records where actual names can be put to comments - were held back, possibly fifty years. The idea presumably was so as not to cause difficulties for those still alive. The irony was that it was quite easy to identify some of these personnel by the jobs they did - indeed often someone, a researcher I guessed, had helpfully written the name in.
I would imagine that if interviewing personnel reluctant to talk about their experiences because of having signed the Official Secrets Act, you would need some sort of official letter to convince them it's OK. I presume that's what happened when all the Enigma stuff came out in the 1970s.