I see what you are saying @Bookworm1111 , but I don't think for a minute those two would be operating the sort of set up OS was describing, or allow some random assistant to open their mail from Buckingham Palace.
OS could be talking himself into a corner here. His adamant denial that his source material for the letter is H&M leaves only one other place it could have come from; and he's therefore relayed the contents of personal, private correspondence, when neither author of the correspondence gave permission for its release. That's potentially the misuse of private information, and actionable.