Whereabouts are you?
Cake Boy by the river in Battersea is worth a visit, but not the trad hotel-style afternoon tea. For that somewhere like The Bingham in Richmond is good. Great views down to the river. Have heard others recommend Cannizaro House, Petersham Nurseries... In central London a friend reviewed all of them and recommended The Four Seasons (which is great), and The Dorchester. Fortnum & Mason is also very good. For something a bit more informal I love the teapod cafes in SE1; and I've also arranged an afternoon tea for a hen do at one of the Tea Palace places, which was good.
I really didn't like the chocoholic tea at The Hilton, which I was subjected to on another hen do