Have you checked the number of years married in 1911, are there any other Bibby / Blackwell marriages that could fit that census entry.
Birth registrations for the other 2 children .....
Mar qtr 1908
BIBBY, Edward
Holborn 1b D/23
note on freebmd - This entry appears to be a late entry referring to the following
Births 1923 Dec qtr
Bibby,Edward, mmn Blackwell,
Holborn,1b,842
BIBBY, John William mmn BLACKWELL
1910 Dec Quarter in ST GEORGE HANOVER SQUARE
Volume 01A Page 365
There are service records for a William Hopkins Bibby for WW1 but they do not mention his wife as next of kin but his mother. His occupation on these records is 'agent'
If they are no longer together or the children are not with him the 1921 census will not help you.