Traditional afternoon tea is a selection of the finest teas and is served with a choice of sandwiches and scones. The Afternoon Tea tradition started in 17th century, and from being a means to still hunger between early lunch and very late dinner, it turned into a social occasion loved and supported by most British families nowadays.
Use the opportunity of being in London and experience that mix of traditions and modern-day trends London is famous for. Most good hotels will offer traditional afternoon tea, but if you don't have a particular spot for London Afternoon tea in mind, consider going to Grosvenor House or Fortnum&Mason.