The Sunday Washington Post travel section ran an interesting article by a writer frustrated that her fall trip had to be cancelled. She outlined the itinerary and noted that many of the places she planned to visit have Youtube channels or short presentations about them.
In London, tea at Claridge's, of course, and the National Gallery, the V&A, and Theatre in the West End. However, she then went on to describe Open House weekend in September. This is an annual event when many important buildings normally closed to visitors are open to them. Lloyd's of London, New Scotland Yard, Lambeth Palace (residence of the Archbishop of Canterbury), and St. James' Palace, which is often the set for films and TV series, including The Crown. Also included is the elaborate Foreign, Commonwealth and Development Office (their State Department), and others. She also mentioned a youtuber historian/actor who posts walks around Mayfair under the moniker "Joolz Guides". At Westminster Abbey, one can visit the Queen's Diamond Jubilee Galleries, which opened in 2018. It houses "art and artifacts long in storage" (check their Youtube video for details.) She also recommended a particularly interesting tour of the Abbey by London's certified Blue Badge guides while they were idle due to the lockdown.
North of London is Chatsworth House, which she describes as the "premier stately home in all of England", It's where the future King Edward VII hosted shooting parties.
There are others mentioned, but she also planned to visit Sandringham, where the Queen normally spends much of the winter, including Christmas. (But won't this year.)
Some of these were new to me, even after 4 or 5 visits to London, so I thought others might enjoy these leads.