Many accurate 1921 census transcriptions have already been added to the pub history site, including London, and parts of Middlesex. If you can contribute to the site, send me some details to Kevan, along with some description of the specific property.
St Giles pub history index
Belton street later appears to become Endell street.
A listing of historical public houses, Taverns, Inns, Beer Houses and Hotels in St Giles in Fields, London. The St Giles in Fields , London listing uses information from census, Trade Directories and History to add licensees, bar staff, Lodgers and Visitors.
Residents at this address
1827/Thomas Pritchard, Coach & Horses, Belton street, Long acre/../../Pigot's Directory
1832/John Sears/../../../Robsons Directory
1833-34/Jno Cumins/Coach & Horses, 21 Belton street, Long acre /../../Pigots Directory