We should remember, too, that, in the first decade after the War, upper class or professional Britons tended to think of Americans as amusing fellows, but lightweight. A favorite pastime among "silly asses" I observed at that time was, "Let's find a Yankee cousin, and see if we can take the micky out of him." I was forewarned on several occasions to expect that kind of behavio, by friends I made.
One can also see it, in interviews of the time, when an Englishman says, in true astonishment: "You know, this American chap was really AWFULLY bright!"
In this regard, remember, too, that the gods of the British Theater, whom Callow would have looked up to as a young actor, didn't think much of Welles in the acting department. For instance, Olivier said or wrote, in a number of places, that Welles could never "build a performance." And Ralph Richardson seemed to agree with him.
We shall just have to wait to see what Callow writes.