Sorry, Nigel, but you are well aware of the concept of constructive dismissal. For example, Wiki express it it as this:

In employment law, constructive dismissal, also called constructive discharge or constructive termination, occurs when an employee resigns as a result of the employer creating a hostile work environment. Since the resignation was not truly voluntary, it is, in effect, a termination. For example, when an employer places extraordinary and unreasonable work demands on an employee to obtain their resignation, this can constitute a constructive dismissal.

(There are lots of equivalent expressions elsewhere)

Trump behaved as he often does. Boris, given a choice of clearly supporting the ambassador at the risk of crossing Trump, went with Trump. The ambassador found that the hostile environment spoken of above.

But I also notice that the answer you gave made no mention of sovereignty, which was an important part of the question.