I've just come across the catalogue of cartoons from Punch magazine.
There are few cartoons about mills (perhaps it not a very cartoony
subject!), but it does include one great image:
Watermill cartoon by Rowland Emett, Jun 1946, where as the
boat is just about to go over the waterwheel, the occupants are
discussing that "...and we should soon be coming to an old mill
which they say has been turned into a fine up-to-date place".