Activity: Brigit's Bakery. Tube Stop: Charring Cross. Cost: Varies. Hours: Multiple times.
You can't leave London without getting real afternoon tea! Brigit’s Bakery is so cute and perfect if you have teenage girls in your travel party. They offer a couple options: there's afternoon tea for 32 pounds, but there is a Tea Bus Tour for 45 pounds! All the food is included with the tour, so it's a pretty good deal. They also have specials around the holidays. I've seen some videos of the interior of the Tea Bus Tour and it looks adorable. It stops by all the famous monuments including some you may not have seen yet. Make sure you check to see where your Tea Bus Tour is leaving from as there are a few different leaving points.
If neither of those things suit your budget, you can just show up and have a cup of tea and a pastry and take your perfect London picture in front of Brigit's Bakery.
Other options for High Tea: the gift shop at Kensington Palace (cheapest option) and the Orangerie near Kensington Palace. I’ve also heard good things about the basement of Selfridges, though I wasn’t able to find the tea shop there. Let me know if you find it!