Probably the best afternoon tea you can have in London but it's a bit touristy. Also, you need to book ahead. However, it's the only tea room, I know of, that has a savoury afternoon tea option which I love.
The Diamond Jubilee Tea Salon at Fortnum & Mason, Fortnum & Mason, 181 Piccadilly, St. James's, London W1A 1ER, United Kingdom