It might depend upon your budget!
It would probably be difficult to beat the offerings of some of the big hotels, such as The Ritz
https://www.theritzlondon.com/dine-with-us/afternoon-tea/
or that at the Royal Albert Hall (which I've often rather fancied myself)
https://www.royalalberthall.com/tickets/events/afternoon-tea/
but you need plenty of money for such places!
Somebody kindly gave me a voucher for afternoon tea (for two) at Patisserie Valerie though and both my friend and I were really impressed with everything there. If I was seeking a reasonably-priced afternoon tea in central London, I might well head for their branch at 55 Wilton Road in Victoria/Pimlico. (If you're going with a friend, buy a gift card online first, to save yourself some money:
https://www.patisserie-valerie.co.uk/products/afternoon-tea-experience-gift-card )