I understand that some people aren't fans of these upper-class movies, and I frequently find myself among them, though I can usually tell pretty quickly whether or not I’ll buy into the premise. When I say “posh,” what I mean is just that—films adapted from works by someone as undeniably posh as Stephen Fry, for example. This one follows a sharply sarcastic main character who's handsomely compensated to look into sup…