Orders? Diplomatic service (in the UK and other democratic nations) isn't part of the military; it's a civilian job. The guards might be soldiers, but the rest of the jobs aren't to my knowledge.
If someone is offered a particular posting but chooses not to accept it without good reason then I expect they won’t be offered anything better (more important, prestigious or more enjoyable).
(Saying that, I remember reading once that high-flying UK diplomats might be offered something less desirable as a ‘test of nerve’ - to see whether they had enough confidence and self belief to hold out for something better. It’s a never-ending test of character and ability to know which way the wind is blowing, I suppose)
