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
The address would have been close to the junction with Little Earl street (later part of Earlham 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/Benjamin Baker, Kings Head, Monmouth street, St Giles/../../Pigots Directory
1829/B Baker, Kings Head, 2 Monmouth Street, St Giles/../../Robsons Directory
1832/C Rich, Kings Head, 2 Monmouth Street/../../Robsons Directory
1833/Charles Rich, Kings Head, 2 Monmouth Street/../../Pigots Directory
1836/Solomon Rich, Kings Head, 2 Monmouth Street/../../Pigots Directory
1839/Solomon Rich, Kings Head, 2 Monmouth Street/../../Pigots Directory
1841/John Gannaway, Kings Head, 2 Monmouth Street/../../Post Office Directory
1841/John Gannaway/Publican/25/../Census
1841/Sabina Gannaway/../20/Middlesex/Census
1841/Sabina Gannaway/../1/Middlesex/Census
1841/William Robinson/Male Servant/20/Middlesex/Census
1841/R Morris/Male Servant/20/Middlesex/Census
1841/Elizabeth Ryland/Female Servant/20/../Census
1842/John Gannaway, Kings Head, 2 Monmouth Street/../../Robsons Directory
1843/John Gannaway, Kings Head, 2 Monmouth Street/../../Kellys Directory
1845/Henry Moss, Kings Head, 2 Monmouth Street/../../Post Office Directory