Hi, Tanja,
The info shown on any cert is only what is told to the 
Registrar by the person registering the event.  How is the Registrar to know whether the informant is lying thro' their teeth.
I have 2 instances in 2 different lines in my own research.
1)  A couple together on the 1851 census shown with the same surname, both very early 20s, no children so presumed to be newly married.   They were tracked thro' all the censuses together with children added as the years went by, 12 in all.   BUT no marriage was ever found for them until.....wait for it.... 1894 !!!!  I nearly fell off my chair when I found that one.  I think perhaps the only reason they married then was that the "husband" had his own business & felt that his"wife" should be able to claim any estate on his death, he died within 3 years of the marriage.
2) my maternal grgrandparents had their first child, my grandpa, in 1869 & the last of nine children in 1887 but no marriage has ever been found.
In both of these cases the mother on the children's birth certs always stated the mother with the father's surname with formally her own maiden name   In both cases the "husband" died first & the deaths were reg'd by the "wife " with the father's surname.  I have come to regard these as "common law" marriages & given up trying to find something that is not there.  To compound it all my own mother used to call her "Grandma" as Granny " husband's surname."
I cannot understand why we can find no census info for Joseph Johnson.  I have found  one that might fit in 1861 when Joseph would have been 4 ? years if born in 1856.
HO107/247  fol 38  page 24
8, Mansfield St., Kingslnd Rd, Hagerstone W, Shoredtich
Charles R Johnson  ahe 50  Messenger b Essex Romford
Sarah wife age 43  b  Middsx Blackwall
Mary Ann day age 20  Fancy Brush maker b London
Emma  dau age 18  b  ditto
Charles  son age 16  b  ditto
Sarah  dau  age ?  b  ditto
Joseph  son age 4 b  ditto.
I am now trying to track this family in the 1871.  Watch this space.
Meliora