If the 1851 is correct then John was living with Jane for at least a year before they married. Jane died 1853 and John married Harriet Edwards in 1854
The 1852 marriage cert shows John's father as a servant
The 1854 marriage cert shows him as a coachman
On both certs John jnr is a servant
The 1841 has a John McCulloch aged 29 b Scotland occ Male Servant living in St George Hanover Sq which is the birthplace of the 2 children on the 1851 entry