Afternoon tea is a light meal taken between about three and five o'clock, consisting of finger sandwiches, scones with clotted cream and jam, small cakes, and tea. It was popularised in the 1840s by Anna, Duchess of Bedford, who found the gap between lunch and dinner intolerable and did something aristocratic about it. It is not "high tea", which despite the grand-sounding name was the working family's early supper, eaten at a high table rather than a low one. Order "high tea" at a smart hotel and you will be gently corrected, or worse, silently forgiven.
The order of things
One works upwards through the tiers: sandwiches first, then scones, then the sweet things. Nobody will arrest you for starting with the éclair, but they will notice. Sandwiches are eaten with the fingers, which is why they are called finger sandwiches and not fork sandwiches.
Milk first or last?
Milk goes in after the tea. The habit of milk first began, the story goes, to protect cheap china from cracking; putting it in first therefore suggested your cups could not take the heat. With today's porcelain it is simply custom, but custom is rather the point of the whole exercise. Pour the tea, add the milk, and if someone asks "shall I be mother?", they are offering to pour, not proposing adoption.
Stirring, and the little finger
Stir gently back and forth, from six o'clock to twelve o'clock, without letting the spoon ring against the cup like a dinner gong. Then place the spoon on the saucer, behind the cup. As for the raised little finger: a myth, and worse, a vulgar one. The finger stays down. Hold the handle between thumb and forefinger and let the other fingers follow with quiet dignity.
The scone question
Halve the scone with a knife, then dress each half as you eat it. In Devon the cream goes on first with jam on top; in Cornwall the jam goes first and the cream sits above it. Both counties consider the other's method a form of civil disobedience. Choose a side, apply it with conviction, and never, under any circumstances, sandwich the halves back together. As for saying the word: some rhyme it with "gone", some with "bone". Rhyming it with "gone" has the better claim in polite society, but you will be understood, and gently placed, either way.
Small courtesies
The napkin goes on the lap, not the collar. The teapot is shared, so an eye on your neighbour's empty cup does you credit. And conversation, not the sandwiches, is the actual point of the meal. The food is the excuse; it always has been.
The nicest way to learn all this is to do it. Eddie's London Experience includes afternoon tea taken properly, in person, with the scone question settled once and for all. Or begin with the English & Etiquette lessons online.