Artsy Experiences in London

Great ideas there, sounds fun! What about Chelsea Physic Garden, perhaps check their website to see if there’s much to see on October. If not, Kew will still have plenty to enjoy.
Also, (probably) the best bookshop in London is Foyle’s, a real landmark. They have a coffee shop/restaurant on the top floor, and a massive collection of books on all sorts of topics. Also Waterstone’s on the corner of Southampton Row and Torrington Place - it’s near one of the universities so it has much more range than most other branches.

I'm not sure if they still have tea dances at the Waldorf hotel.
Harrods is a zoo.
Regents Street or Covent Grden for uoscale/interesting clothes.
Whittards for teas.

Hatchards bookstore in Piccadilly is my happy place. And it's right next door to Fortnum & Mason, whose ground floor is a good place to browse for tea, chocolates, and biscuits.

Barnaby Bars if you like chocolate. I walked by (and eventually walked in) their Covent Garden location last year. Pretty darn good.
https://barnabybars.com/

Sounds like a nice trip, here are some additional suggestions from me:

-Gardens- Regent's Park (Queen Mary's Rose Garden), you might also be interested in the Garden Museum (excellent cafe)
-There's not a whole lot on fashion in the Design Museum that I can remember, I think Fashion and Textile and V&A are your best options. I might browse Liberty rather than Harrod's. It's well known for British design.
-You can check out London Walks to see if they have any ghost type tours. There is some kind of ghost bus tour that I've seen driving around but I don't know anything about it. There are plenty of haunted places in London.
-You'd probably like Spitalfields for clothing shopping,
-For tea- Fortum and Mason, both for buying it and for drinking it- for me it's the best tea in the UK. For gin- you can do a tour/tasting of Sipsmith in west London, or visit the Beefeater distillery near the Oval. Sipsmith is a bit more "boutique." For whisky tasting, I suggest Scotch Malt Whisky Society: https://smws.com/venues/19-greville-street-london
-Two very nice book shops- Daunt Books on Marylebone High Street and Hatchard's on Piccadilly. Lots of places to read tho in October you'll probably want somewhere indoors.
-National Theatre tour is very good. You can also tour Royal Albert Hall.
-There are so many music venues- I would consult Halbut's: https://halibuts.com/

Good luck with your planning.

Been visiting and staying in London since 1972. Always changing, always the same.

Kew Gardens is huge.
Check the Rivoli Ballroom schedule for your travel dates
Liberty and Selfridges department stores
There’s a great hat vendor at Spitalfields
Spitalfields should have much of you’ve listed
Fortum and Masons for tea ( costly but well done)
Isabella Plantation in Richmond Park is lovely in the Fall
My Favorite is Hurlingham Books. I’ll make a day of tubing there ( Putney Bridge Station ) getting breakfast or lunch at the River Cafe ( opens at 6am ) or a strong double expresso at the teeny Dr Expresso Caffeteria. Hurlingham Books doesn’t open til 10am so you could tube to Parsons Green Station and explore the area on foot walking there.

Lastly check the London Walk website. I take one walk each visit. Last one in 2024.

I would also recommend Spitalfields for clothes shopping. There is an indoor vintage market on Brick Lane at the Truman Brewery which is all in the same area.

For books, you could also check out Waterstone's on Piccadilly. It's the largest bookstore in Europe and only a few minutes walk from Hatchards. It has a couple of coffee places in the store as well as a bar/restaurant on the top floor.

I used to recommend the old Foyle's books with all its little nooks to get lost in. But the new store is modern and organized.

For edibles, I second the aforementioned Fortnum and Mason. A stop at the Harrods Food Halls could be of interest if you have the time.

But if you really want a unique food experience, check out the Marylebone Farmer's Market. It's run every Sunday and considered the poshest in London. I haven't been....yet....but I was told to bring lots of money if I want to buy things.