Historically, tea was poured into the teacup first - Tea in First (TIF); this follows Royal Protocol.
Sugar was added to most tonics or herbal beverages in the late 1700s for extra sweetness; the imported Chinese tea was bitter and usually stale after long sea voyages and sugar helped lift its taste.
Thinly sliced lemon adds extra refreshment to most black teas and is usually offered on a dish near the milk and sugar. It is, however, most irregular to add both lemon and milk to tea.
The fashion of adding milk to tea started in 1760; milk was poured in first to stop the English porcelain from breaking due to the heat of the tea
By the 18th Century, Industrial Britain needed milk and sugar to fortify the workforce. By the 20th Century tea had become a national passion.