The Origin of High Tea

This article comes from Wikipedia

Tea in England was initially served in coffee houses. Due to high taxation it was expensive, and only affordable for the very wealthy. Despite the cost, tea drinking became widely popular, and tea sellers such as Thomas Twining started selling dry tea, so that ladies who could not frequent the coffee houses could enjoy it.

Tea was very valuable, and was kept by the lady of the house rather than in the care of the housekeeper. It was the lady of the house also who would serve the tea, in imitation of the Japanese tea ceremony.

The following is disputed, and may in fact be an urban legend:
Anna, 7th Duchess of Bedford had the idea of asking her butler to bring tea, bread and butter to her chambers at 5 o'clock, as she found herself hungry before dinner, and soon started inviting her friends to join her in her sitting room for this new social event. Eventually, the beverage tea became generally affordable and the growing middle class imitated the rich and found that the meal tea was a very economical way of entertaining several friends without having to spend too much money, and afternoon tea quickly became the norm.