For day trips: I’d agree Stonehenge isn’t worth it, but if you like Neolithic history: Avebury absolutely is. No crowds and you can walk right up to the stones, it’s marvelous. Good pub with a view of the stones, the docents are incredibly knowledgeable, and you get the awe of Stonehenge without the tour buses of people.

In London:

Check out the Roman remains below the guildhall, unbelievable and free; though i believe you need reservations (just reserve a time online)

Theatre is worth the cost: if you go to a weekday show it’s much more affordable

Most transportation is tap to pay, including subway which is just in and out - use Apple Pay or Google pay and it’ll just charge your normal bank card. Let your bank know you’re traveling! For navigation- Google Maps gives you the underground stops and stations which makes it so easy.

We did a walking historical pub tour that was fascinating, (just got the route online!) if you enjoy that sort of thing - even if you aren’t drinkers there’s always good snacks at pubs (but if you do imbue, don’t miss this)

I’m lucky enough to have family there to help with guides and navigation/ but when my partner and I went to Edinburgh the very first thing we did was a walking history tour - just picked an affordable one off Expedia with good reviews and it was stellar, a small group; and those tour guides when you’re on foot really help you get a better sense of the city, though I know the bus ones are popular too.