There is a big difference between afternoon tea and high tea. Afternoon Tea will begin sometime between 2 and 4, and is the most formal of teas. (You will definitely want to dress nicely for an afternoon tea.) You will be served black tea (possibly a choice of two -- perhaps plain black and an Earl Grey, which is black tea infused with oil of bergamot, a bitter citrus), and three courses of treats -- fruit, dainty tea sandwiches, and pastries. There will be sugar and milk provided (never cream, which is considered much too heavy for tea), and the custom is to pour the milk into your cup before the tea. Sugar will be in a bowl (God forbid paper packets!), and will either be lumps (cubes) or loose. There will be tiny tongs if lumps, and a spoon if not. Do not put the spoon into your tea! Pour in the sugar with the sugar spoon and stir with your own spoon. (Your teacup and saucer will have a small spoon with it.) The food may be served on a small tiered tray, with tongs to select what you want to put on your small plate. If you choose to wear a hat and gloves, women may leave their hats on (not men) and all gloves MUST be removed before eating!
A High Tea, which sounds formal to Americans, is not. Many years ago, when tea first became available to the masses in England, they wanted to drink it, but had to work all day, and could not stop to enjoy tea (or any meal) in the middle of the day. When they were done in the fields, they would come home, but of course typical tea fare was not enough food after a long day. The lady of the house would prepare a large meal, but it was far to big to serve on a "tea table" (which was a tiered table developed just for serving morning or afternoon tea and the accoutrements), so it was served on the "High" table a.k.a. dining table, and obviously became known as High Tea. High Tea is a working person's tea, and includes enough substantial food to call it dinner. If you opt for a high tea, it will be served around 7:00P, and there will be hot dishes.
If perchance you wish to attend morning tea, it will be similar to afternoon tea, but not quite as formal and no sandwich course, just fruit and pastries. Breakfast tea will be served, which is a combination of two different India teas and a bit stronger, malty taste than tea served later in the day -- kind of an eye-opener.
Hope this helps.