Been to London very recently: https://www.furaffinity.net/journal/10536498/
Have done the touristy stuff mostly, lol. Would say Madame Toussauds is really worth it. Amazing what they created there.
Otherwise if you like that music style (and are up to spend the money), the Abba Voyage is a really unique experience.

For food I'd have liked some recomendations back then too. It's surprisingly hard to find something good q_q

Checked out your journal and while the stuff you did is mostly stuff I've done already or aren't personally interested in, it certainly sounds like a fun time. Truthfully, being British myself and living in a coastal part of Scotland where fish and chip type pubs/take-aways are a dime a dozen I'm far more excited to eat at a bunch of restaurants serving different cuisines rather than traditional 'British' stuff (I airquote it because a lot of 'British' restaurants are just using a tonne of French/Italian classic techniques anyway, nothing wrong with that but it always makes me chuckle) so I'll be less seeking out tea and fish+chips and more seeking out ramen, curries and different street foods from a bunch of the city's markets. :3

If you want a *manageable* art gallery that’s not too big, busy or gaudy, the Courtauld Gallery in Somerset House near Waterloo is great.
Food wise, Kanada-ya in Soho for ramen, Kintan in
Oxford Circus for yakiniku style meat. But then I’m biased as I just love Japanese food. Enjoy your time there!

I may or may not have a list of about a dozen possible Japanese restaurants, most of them ramen joints, that I want to try at least a handful of. :D Kanada-ya is already on my list, but I'll definitely check out Kintan too!! And thankee for the Courtauld recc, I'm actually staying close to Waterloo so that'll be super easy to visit. :o

Not sure if you would consider this one of the major tourist spots or if you've already been, but HMS Belfast is neat if you are interested in naval history.

Definitely something I've considered, but truthfully my historical interests lay more in more ancient stuff than 20th century warfare! Great suggestion though, even if it's not something I'll probably end up visiting this time round.

If you want ancient, there are a few Roman remains visible here and there.

https://freetoursbyfoot.com/roman-london-ruins/

* nods * I know about them already, thankee though!

Camden Market has suffered from the pandemic driving a lot of the stall holders out of business, but it's still fun to look around, and there's a wide variety of food vendors to choose from.

A lot of markets are on my to-do list, and Camden is certainly one of them! :3

Check out Youtube channel Sorted Food. While they mostly do just in-studio cooking videos, they used to do a lot more of showcasing for food in London. Look up their "London's Best" series of videos to get some ideas for those. The one place that I remember them bringing up repeatedly is Pizza Pilgrim being in their opinion the best pizza in London, so they look like a definite checkout.

When I was first looking up places to go in London food-wise I was combing youtube for different things and definitely watched a couple of their videos, haha! :D

If you haven't done the Globe Theatre - and can stand for the play rather than sitting down - paying a fiver for one of the performances is really enjoyable. Depending on when you're down, you could see A Midsummer Night's Dream, Macbeth, or As You Like It. I saw The Comedy of Errors recently, and that was amazing, and the Globe usually does a good value for money with its performances.

I have it in my head that if ever I'm in London while a production of Twelfth Night (my fave) is on, I'll definitely see it! :3

They have several filmed and records that you buy or download.

* nodnods * I've seen several different productions of it in recorded media of various sorts (my favourite version so far has Parminder Nagra as Viola and Chiwetel Ejiofor as Duke Orsino, though the Imogen Stubbs 90s version is great too) but I'd really like to see it live in person at some point. :3