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.
By visiting the relevant London Public Houses, you will find Licensee information, bar staff, boarders & lodgers from Census and Trade Directory information.
Visit the London pub history page for many other London directory listings.