Being too young to have seen the original advert, I don't have much to add, except that I came to know it only through parodies and was utterly baffled, when I first heard the real thing, to discover that it was not a Yorkshire accent. I wonder where the Two Ronnies parody comes in the history: https://www.youtube.com/watch?v=DJi_5T0jSnA (the little - surely impromptu - reassuring pat of the loaf of bread is a reminder of how ineffably good a comedian Ronnie Barker was though).