had been servants, was a bit more realistic in this regard.
The Bellamy family treat their servants as manipulable objects and monitor their personal lives. ("I do not allow my servants to marry" Lady Marjorie says at one point.)
Furthermore, there is a hierarchy among the servants. Hudson, the butler and therefore highest ranking of the male servants and Mrs. Bridges, the cook and therefore highest ranking of the female servants, reinforce class values. For one thing, they are always "Mr. Hudson" and "Mrs. Bridges," while the lower-ranking servants are addressed by their first names. They continually remind the other servants always to have a respectful attitude toward their "betters," by which they mean the aristocrats.
Interactions between the servants and the members of the Bellamy family are mostly business-like. Lord Bellamy tells Hudson to get everything ready for a trip to Scotland, and it happens. Lady Marjorie tells Mrs. Bridges to prepare a dinner for the king, and it happens. Later on, when Georgina, inexperienced at dealing with servants, treats them as equals, it makes everyone uneasy.
Yet the servants have devoted their lives to the Bellamy family at the expense of having no lives of their own. One tragic figure is Lady Marjorie's long-time personal maid, who survives the sinking of the Titanic while Lady Marjorie does not. She turns up on the Bellamys' doorstep, because, as other cast members note, she has no other home. But she also has no further role, with Lady Marjorie dead, so what is to become of her?
In the 1970s, when Upstairs Downstairs was broadcast, there were still a lot of people alive who remembered the old pre-World War II system of aristocrats and servants. Any series that showed the aristocrats being buddy-buddy with their servants would have been laughed at.
Forty years later, fairy tales about kindly, egalitarian aristocrats are more widely accepted.