Very, very popular with locals, reservations are best made 90 days in advance as soon as bookings first open: https://www.lux-review.com/2015-afternoon-tea-at-claridge-s/

I always do at least one, high end afternoon tea every time that I get to London. Although Claridge's was a very pleasant afternoon, it was not the absolute best afternoon tea I've experienced in London. I'm still glad I was able to do it. Because I frequently travel on short notice, I seldom have the luxury of booking 90 days in advance. This time, I did. Now, I know what all the hype is about. It was a relaxing late afternoon.

Olivier Gagnère of Paris, France designed the iconic green and white striped tea services used at Claridge's.
Dale Chihuly of the USA designed the curled Chandelier for Claridge's Foyer "tea room".