When I moved to London for the first time, in 1989, the crypt at St Martin in the Fields, Trafalgar Square, was one of the first places I went, and often. The food at the cafe was cheap, you could nurse a coffee for hours, the surroundings simply oozed history, it was quiet and serene, and there was the added thrill of walking over dead people. Well, walking over old, worn, carved stone panels in the floor memorialising dead people, anyway.
During the 15 years I lived in London, I returned many times, often in conjunction with a concert in the church above (in 1991, I attended the concert to celebrate the church’s new organ, “in the presence of HRH The Princess of Wales”, the ticket words it; as I recall, we had an encore singing of “Jerusalem” at Diana’s request).
The crypt has changed over those years, less a hidden gem now and more another tick on the tourists’ bucket list, but the history and the architecture is still there — as is the thrill of walking over dead people.
If you’re curious about the church: It is dedicated to Saint Martin of Tours. There has been a church on the site since the medieval period. The present building was constructed in a Neoclassical design by James Gibbs in 1722–1726. (source)