Seeing as it's Fortnum, I assume you mean "afternoon tea", not "high tea" (which is a generally a family meal taken in the very late afternoon/very early evening). Their menu looks lovely, if eye-wateringly expensive.
I also assume you mean afternoon tea. Fortums is very good for this. I went for a family occasion a few years ago, and it was really wonderful. Lovely sandwiches and scones, and a seemingly unlimited supply of cake (they actually gave us extra cakes in a box to take home!).