What to wear

Ask them?

https://support.fortnumandmason.com/hc/en-gb/requests/new

Here is Fortnum's response from their website:
https://support.fortnumandmason.com/hc/en-gb/articles/360002781977-What-is-the-dress-code-for-the-restaurants-

The restaurants include the tea room. When I’ve been there on a weekday in fall and winter, small groups celebrating birthdays were a little dressed up. The Instagram world was dressed to post. And the rest of us seemed to be wearing tailored slacks and simple tops.

Depends on what type of occasion you want to make it. Fortnum embraces sneakers and more casual attire, including shorts.

If I was going there, no way would I wear trainers (sneakers) or shorts. Going there is an occasion for most people and I would therefore smarten up. You want to fit in, presumably, not shout “tourist”.

You may want to know that "high tea" refers to the meal at the dinner hour eaten by servants seated at the table with high-back dining chairs. Afternoon tea is the light afternoon meal enjoyed at what we Americans might call a coffee table. https://www.thespruceeats.com/afternoon-vs-high-tea-difference-435327

“ we Americans might call a coffee table.”

Huh? Yank here.

Coffee table is a piece of furniture.

Coffee clatch is the grouping to get together and have coffee.

I apologize if my mention of a coffee table was in any way confusing or off base. What I meant is the difference between a low table, like a coffee table, where afternoon tea would traditionally be taken. Think of the low table between the sofas in the library at Downton Abbey where we see the family members having afternoon tea. That low table is in contrast to the dining-height table where the servants have their tea (i.e., high tea, or what we'd call supper or dinner).

whenever i have gone for afternoon tea it has been at a normal table with normal chairs. I guess I'm not highbrow enough for low tables.

This afternoon we had tea in Shropshire - regular height table for two.

Thank you for the help

We loved tea at FM-it was quiet and relaxing. We wore business casual clothes and I'm sure I wore shoes that are the same type shoes as tennis shoes, but not white.

I agree that unless a place says a specific dress code, just wear what is comfortable for you. No one is looking at your clothes or your shoes! We liked the Willy Wonka tea ourselves. My husband wore jeans and a button shirt. I wore a sundress.

F&M is fun to just explore as a store. Grab some food and eat at the local Green Park if the weather is nice.