This has to be one of the more unusual afternoon tea in London experiences I've had the pleasure of sampling. The idea was dreamt up by BB Bakery, who have a permanent tea and cake establishment in Covent Garden.
The afternoon tea bus tour starts and ends at the London Eye and takes in Big Ben, the Houses of Parliament, Buckingham Palace, Harrods, the Royal Albert Hall, Hyde Park, Marble Arch, Piccadilly, and Trafalgar Square. The map is helpful stuck to the table, along with several suckers, as pictured here, which help to stabilise your plate. (Note, there are no toilets on board and the tour is lasts more than an hour, so go for a tinkle before you embark).
Due to the nature of being sat on a moving vehicle, there is a certain amount of toing and froing so don't expect this to be a smooth ride. The tea is actually served in coffee shop style lidded mugs so that hot tea isn't splashed all over your nice new outfit. There's also a handy booklet to accompany your tour so that you read up on all the main sites that pass by. There is no loud speaker commentary, so idle conversation is encouraged.
Don't worry if you don't eat everything. You're given a little doggy bag at the end of the tour.
Be prepared to have your photo taken quite a lot. There will be many people outside the bus peering in at you, jealously, with cameras in hand. Give them a wave. Make them smile.
For a more tranquil, stationary experience, there are other afternoon teas in London.