This question comes up periodically. The places that offer afternoon tea don't change too much (I believe the Taj is still the most recent entrant), so previous posts on the board are likely your best bet:
I took my 16 yr old last March to the Four Seasons (Bristol Lounge) and it was fantastic. The service was great and so was the tea. The food was okay as well. They did not rush us and the seats are really comfy. We had a great relaxing time and it was a great treat.