Clearly you have been watching too many episodes of Downton Abbey(《唐顿庄园》). Anyway, afternoon tea is a posh(aristocratic/top 0.1% of the population) snack (as we non- aristocrats would call it) between lunch and dinner that was accompanied by a cup of tea. We regular British people drink tea and eat snacks whenever we feel like it-8 a.m. 9 p.m. 2 a.m. In my house while I was growing up, we would have a big dinner at lunch time (weekends mainly) and then tea later. This would consist of a sandwich and a piece of cake or a biscuit that we ate in front of the telly(电视机). We were too full from the enormous dinner (called lunch in the US) that all we could manage was small meal. Even now the meal after work/school is called tea whether you drink tea with it or not, and whether it consists of sandwiches or is a main meal.