We enjoyed a Context Tour of the National Archeological Museum in Athens in June and it was excellent! A little pricey but among the best tours my husband and I have ever done. I believe they only offer private tours.
We did the Pristine Sistine group tour of the Vatican Museums with Walks of Italy in 2016 on the recommendation of many on the forum. They offer many other tours in other cities as well.
https://www.contexttravel.com/cities/london/tours?end_date=2022-12-09&pax=2&start_date=2022-11-25