All very interesting, and it does raise a couple of questions:
1. The light and heat over Boris Johnson's refusal to "endorse" Darroch is a bit disingenuous in my book. Once the Daily Mail published the leaked email's, Darroch's fate was entirely in the hands of the White House.
There was a previous occasion where a newly-appointed Ambassador had, in his past career as a journalist, been very critical of the incoming US President (Nixon, possibly, but don't quote me). The President's reaction was to shrug and say "Well, he's a new diplomat and I'm a new statesman" and there the matter lay.. Trump's reaction of course was entirely the opposite, and whether you agree with him or not, it was completely his call to make.
If Darroch decided to wait to see if he would be "endorsed", by Boris or whoever, that shows poor judgement on his part in my view. Once it was obvious that the White House were going to black-ball him, he should have fallen on his sword and resigned immediately.
2. Looking at the leak itself, it will be fascinating to learn the source. If it's not a state-sponsored hack, which I wouldn't rule out (because it was obviously a long term effort over a couple of years; those emails were supposed to self-delete after 3 months), it's got to be an internal leak, a Bradley Manning / Julian Assange scenario, and that has to be an offence under the Official Secrets Act. I cannot see anyone handling those emails as not having signed the Official Secrets Act, can you?
So I think it is entirely right that the Met's Counter-Terrorism unit have been called in to investigate, and I don't have any truck with George Osborne and the other newspaper editors warning of dark and perilous times over the Freedom of the Press. Freedom of the Press should not be an overarching right, it has been abused by the press too often (Cliff Richard, etc.). What do you think?