One of my first projects when I came to London was looking at Churches. As we were staying in Aldgate this was the heart of early London with many interesting early buildings. Some of my favourites were St Katharine Cree, St Andrew Undershaft (next to the old maypole) and St Olave Hart St where Samuel Pepys was buried. These are all wonderfully atmospheric churches with ancient tombs, sculpture and stained glass. My favourite is by the river; St Magnus Martyr, dedicated to a Viking saint, with roman remains and an enormous model of the old London Bridge, densely populated with tiny figures of people and animals. All Hallows by the Tower is also fascinating mainly because of the ancient crypt full of cryptic (groans) monuments and archaeology. St Martin Ludgate near St Paul's is a wonderful Wren church with stunning woodwork and unusual old furniture. I went to more Wren churches than you can shake a stick at. Many served as the homes of craftsmen's guilds and feature displays of tools or shoes or crafts.
Over the Thames, I found Southwark Cathedral, a wonderful and important building hemmed in by railways and markets and well off the tourist trail, but peaceful and beautiful.
The most impressive and atmospheric of all is St Bartholomew the Great. This unfathomably ancient and gloomy building tucked away behind a half timbered entry (pictured) was part of an early monastery and hospital. The hospital is still extant. It has limited opening hours and an entry fee though.
As to more recent churches, I loved St Mary Abbot in Kensington with fascinating monuments (so many stories) a pretty churchyard and peaceful cloister, surrounded by tiny lanes of quirky shops.
The most mindboggling of all is Westminster Cathedral, a Venetian inspired giant on Victoria Road crammed with gold mosaics. A very kind priest gave me a personal tour which was one of my best church experiences. The worst was a damp crypt lined with coffin plaques and with a repellent, strange smell. I will keep exploring.