The UK Official Secrets act. Very blunt instrument. You don't have to have "signed" and if you do they don't give you a copy of what you signed.
I'm not sure what Compton Mackenzie was thinking to quote from an official document, but I see on the preface he was advised he couldn't sanely plead "not guilty". I've a few of his books on paper, but none of the memoires, though they are the sort of thing my dad collected.
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