Under the 1911 Official Secrets Act, it was illegal for anyone who had been 'notified' that their actions were covered by it to disclose any official information without lawful authority. (There wasn't, and never has been any requirement for an individual to sign a document confirming that they'd received such notification). As the law stood at that time, and up until 1989, it was technically illegal for someone to simply disclose to a mate how much they were charged for a biscuit from a tea lady in Whitehall offices.
In 1989 though the law was revised. Persons who have been 'notified' (often by the terms of their employment) that their actions are covered by the Official Secrets Act can now only be prosecuted where the information they unlawfully disclose relates to
a) security and intelligence ; or
b) defence ; or
c) international relations ; or
d) information which might lead to the commission of crime ; or
e) 'foreign confidences' ; or
f) the use of special investigation powers under the Interception of Communications Act 1985 and/or the Security Services Act 1989.
Where information does not fall within one of those six categories, or where a person hasn't been 'notified' that their actions are covered by the Act, they can't be prosecuted using the Official Secrets Act.