Which of the ambassador's private comments was inaccurate, and which do you consider more "slating" and unprofessional than the public twitter response from Trump in his role as 45th president of the United States?
None of those questions are relevant. Diplomacy requires discretion and deception, even regarding allies. The key word in your post is "private". What a diplomat says publically and what they say privately may well be very different things. That's the nature of diplomacy. Their report to home, which should have been private, was obviously slating. Nobody has said it was unprofessional (quite the opposite, actually - the diplomat was being very professional). Accuracy is also irrelevant - it was the diplomat's opinion, not an objective measurement of anything. Comparing it to Trump is silly. No sane person would call Trump a diplomat. He has about the same level of diplomatic skill as a drunken monkey.
In this case, both the diplomat and Trump are in the right. The UK diplomat gave his honest opinion to the UK government while being diplomatic with the USA government, which is what he should be doing. The president of the USA refused to work with a foreign diplomat who has an extremely low
and publically known opinion of the USA government, which is what he should be doing.
When I signed the Official Secrets Act, I didn’t get told of any caveats like “being in the public interest”.
It was very much “Breach this in the morning and you’ll be in prison by teatime.”
When my father signed the OSA, it was very explicitly an elaborate but very clear statement of "Do Not Talk About This". No ifs, no buts, no maybes, no circumstances in which it didn't apply. You did not talk about it to any person, at any time. You did not talk about it to your dog. You did not talk about it to yourself. Even 50 years later, when it probably no longer mattered, he didn't mention it. Or maybe it did still matter. I only ever had the vaguest idea of what he did. Something to do with cryptography in the military. I never even knew his security clearance, only that he had one. He had agreed to not talk about it, so he didn't. Simple as that.