Cloth House, 47 Berwick Street
Fabric shop on Berwick Street in Soho.
Two locations, both on Berwick Street. Generally have a good range of fabrics and at least one of the shops (can't remember which one but I think it's 47) has a nice range of trim and buttons.
OS X co-ord: 529423 OS Y co-ord: 181264 (Latitude: 51.515481 Longitude: -0.136198)