My Department has published a clearing house toolkit, an annexe to which contains a list of triggers for referring cases to the clearing house. This document is located on the DCA website at http://www.foi.gov.uk/guidance/pdf/annex-b.pdf.
Guidance for making Freedom of Information requests for information from any records in the custody of the National Archives including 1911, can be found in their publication scheme:
http://www.nationalarchives.gov.uk/documents/access_rights.pdf
The 1921 census is in the custody of the Office for National Statistics and not of the National Archives.