Not sure a leak of conversations or unsecure communications through unofficial channels with no appropriate official security markings, would be a criminal offence. It may be a breach of the Civil Service code of conduct, which SPADs including Dommy would've be bound by, but as such no more than a internal disciplinary matter, unless an issue of National Security was involved....
Funnily enough, the CS code of conduct does cover whistleblowing.....and actively encourages it, particularly where the commission of a criminal act is concerned
Looks like he's done his duty for queens and country on that measure