Mr Banks had to work for living. The rich don’t.
That was the urban Edwardian middle class. Only the impoverished couldn’t afford at least a couple of live-in servants.
Not just in England, too. I saw the US Census records for some of my ancestors in the 1910 Census. From all my genealogy research, the Hansen family of Butte, Montana, was solidly middle-class townies, and census records a cook, a stableboy, and a maid as residents along with my ancestors.
I am in awe! 
This would seem to be the definitive answer considering it was written by the creator’s son.