It's all about what goes on in the household of an upper class family. 'Upstairs' refers to the Lady & Master of the house. 'Downstairs' refers to their servants. Upstairs they have 'carryings on'. Downstairs they have 'goings on'.
Historical events or people such as The King, The Suffragettes, The Titanic etc, and 'taboo' subjects such as homosexuality, pre-marital sex, the adherence to traditional roles & everyone 'knowing their place or social standing etc, becomes part of the Upstairs Downstairs subplot.
Lady Marjorie was the aristocratic daughter of an Earl & Countess. By contrast her husband Richard Bellamy was the working son of an Anglican Parson. Therefore 'The Bellamy's' lived as a modest upper class family, in a 'Town House' - 165 Eaton Place, Belgravia , Central London, 'quite' close to Richard's work as an MP in Parliament.
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