>>> presumably all no. 10 staff have signed the official secrets act.
Nobody ever
signs the Official Secrets Act. (It's a myth). People are only subject to the provisions of the Act is they're either members of the security and intelligence services or if they've been officially notified (e.g. as part of their contract of employment) that they're subject to the provisions of the Act.
Even if people are, as above, covered by the Act, it's still not an offence under that Act to leak information unless it relates to "the work of, or in support of, the security and intelligence services or any part of them" or to defence, international relations, 'foreign confidences', 'special investigation powers' or to matters which might lead to the commission of crime.
https://researchbriefings.parliament.uk/ResearchBriefing/Summary/CBP-7422