"I am the “researcher” for our travel group, so I feel obligated to choose a crowd pleaser for our first (brief) visit to London."
Awww sweetie....do NOT take on the obligation of making sure others are pleased. It's tempting especially if you have been there before or if you are more of a traveler than others. Speaking from experience....give your other travelers 3 choices and let them pick. Do NOT let them say "Whatever you decide is fine".
I've no suggestions as the last time I had afternoon tea in London was in the 1980's, hahaha. I DO recommend Betty's in York or Harrogate if you are up that way, lol.