h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Afternoon tea, as opposed to a cup of tea alone, is a meal. In addition to copious amounts of tea (served in china cups with saucers, from a pot), possible components include cucumber sandwiches, salmon sandwiches or ham sandwiches - cucumber perhaps the most 'traditional'.
Scones may also be consumed or substituted for the sandwiches. If the scones are served with cream (clotted cream is best) then afternoon tea may become a cream tea. Jam should always be provided.
A third alternative is toasted bread products. Warm toast or teacakes (fruit buns) or English muffins (not American cupcake style ones) or crumpets all with butter (NOT margarine). Honey and jam can be offered.
However you begin, afternoon tea should also involve cake. These should be English style rather than French style patisseries or gateaux.
Good choices would be victoria sponge, rich fruit cake, madiera, Battenburg, ginger, coffee, date and walnut or fairy cakes (cup cakes with butter icing). Ideally a couple of varieties should be on offer but one will do.
Some cream cakes are acceptable such as chocolate eclairs or small meringues - but I personally don't go for these.
Buns are another possibility - fruit buns and especially Chelsea buns (a bit like a doughy Danish pastry) are good.
Biscuits or even Petit Fours can be served too - shortcake work well as do homemade ginger biscuits. Avoid cookies.

Timing: 4pm is about right. Afternoon tea acts as a fourth meal in the day so don't go overboard and make sure dinner is later.

Some American tourists asked me in the lift at work whether English people had afternoon tea everyday or just on weekdays.
For the record, English people very rarely eat at tea time now, at most a cup of tea might be grabbed but none of the above.
Some people might have a late afternoon tea at the weekend, especially on Sunday, if they had had a large lunch, but this would be a substitute for supper.
Afternoon tea is now reserved mostly as an indulgence when holidaying elsewhere in England and time and the waistline permit.

I feel hungry now.