Then of course there are Secrets and Top Secrets.
Do you think for one moment that Ministers are given access to real secrets? By all means they are custodians of secrets that can be exceptionally dangerous if released to a foreign power. Would we or would we not do this or that, yes they determine that and could betray the trust.
DMPI for a missile or bombing attack, names of agents, how a submarine or aircraft works - none of that detail is necessary for them to do their job. Does the missile work or is the subway serviceable, yes they do need to know. But would Sir Humphrey tell them if they didn't ask?