Around halfway through “Operation Mincemeat” — a busy yet somewhat rousing WWII spy thriller based on the English military scheme of the same name — I began to appreciate why this might be John Madden’s best movie since “Shakespeare in Love”: It’s a story about a bunch of British men (and a smattering of British women) who are trying to stage an elaborate show in the face of escalating crises. Except this time, their audience isn’t the Queen of England, a crowd of rowdy peasants, and a pissed-off Colin Firth. This time, their audience is the Nazi intelligence network, and their lead actor is a pissed-off Colin Firth. And unlike “Shakespeare in Love,” much of this story is actually true.…