Cloth House, 98 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: 529549 OS Y co-ord: 180985 (Latitude: 51.512437 Longitude: -0.132881)