All Souls Church, Langham Place is a striking John Nash church just north of Oxford Circus, with a circular vestibule and a spire that still cuts through the traffic and shopfronts around it. Completed in 1823, it is one of the better architectural surprises near Broadcasting House.
Access can be limited outside worship, so treat it mainly as an exterior stop unless the doors are open. It fits neatly into a walk between Regent Street, Fitzrovia, and the BBC buildings.