I imagine ambassadors advise their leaders to play it careful with Trump. Stay on his good side. Try to accomodate America's needs and complaints. He has a "shoot from the hip" side. On the other hand, he did not attack Iran when they shot down one of our drones. This is all advantageous to America. If you kiss his butt and support America, he'll compliment and support you. If you become disloyal and do things that hurt America, then he'll take a swing at you. Pretty much how he runs his life. Exactly how a American president should operate. I wouldn't expect less from your leaders.
I should imagine nobody tells their ambassador how to behave; knowing how is part of the job description - or selection criteria at the very least.
Conflating drones with Brit ambassadors is a wild one - but hey, it's the Internet!
I had not realised that a foreign ambassador owes his country of duty "loyalty"; I wonder if anyone else knew that? Does your man in Moscow feel he owes Russia a sense of loyalty? Oh, now wait a minute... Certainly that has to be the case in Israel. Gives "family" a warm, familar Italian feeling, doesn't it?
In any case, the ambassador did nothing wrong. The person or persons who did that are still to be discovered. Then, it will either be a public display of indignation or a quiet reprimand and possibly a prosecution. I would sue for treachery if it's on the UK side, and take it as rather stupid if from the American: what else do they expect ambassadors to do but give their opinions, just as do American ones - except of client countries in the Middle East, perhaps. Or maybe they don't and they all do it as you prescribe, so that their nation (America) never gets to hear what the noise of the truth on the ground sounds like. Could explain a lot; extend the fantasy into vital life situations. Cool.
Hollywood and Marlboro Man have never been so important before; now they have become the new reality.