St Bartholomew-the-Great is one of London's oldest surviving churches, founded in 1123 by Rahere, a courtier who became a monk. Its Norman interior survived the Dissolution, the Great Fire, and the Blitz, leaving a rare medieval atmosphere in the middle of the City.
Visitors are welcome outside services, usually with an admission charge for sightseeing. Go for the heavy stone arches, the sense of age, and the sudden quiet after the streets around Smithfield.