We just got back from London
+1 for Greenwich - Maritime Museum and Observatory. The tour guides at the observatory were very good.
We did a canal boat ride that was an interesting bit of London history I hadn't been familiar with.
For HP fans, take a daytrip to Oxford and do the HP tour there - it shows you places around Oxford that inspired things in the books, e.g. there's a statue that students once partially chopped the head off of = nearly headless nick, dining room with the four long tables, the room used as the infirmary in the movies.
We stayed in a flat in South Kensington, near the Glouchester tube station and that worked out great.
https://nestorstay.com/