The only possible marriage I can find for a George Jacobs and a Mary Ann in the 1851 time frame is in Sept.qtr.1851 Lambeth to a Mary Ann Ingram. Their sons birth is in June qtr.1851 (only 1 mo on census) and if they married around same time it is possible it was not recorded until the Sept.qtr.
Have also found an Ingram family in Lambeth.
1851 Lambeth - HO107 - 1570 - 200 - 38
James Ingram 65 Shoemaker b. Salisbury
Sarah Ingram 55 Shoe Binder b. Bristol
Richard Ingram 17 Bricklayer b. Margate, Kent
Jane Ingram 15 Shoe Binder b. St. Johns, West
Frances Ingram 12 Shoe Binder b. Lambeth
1841 Lambeth - HO107 - 1061 - 7
James Ingram 55
Sarah Ingram 45
Henry Ingram 18
Frederick Ingram 15
Sarah Ingram 13
Mary Ingram 11
Richard Ingram 9
Frances Ingram 2
(don't know where Jane was ??)
Cannot find George Jacobs anywhere on 1861 Census - have checked under any permutation you can think of so looks like somehow they evaded the enumerator that night.
Think you need James Jacobs birth certificate to be certain - birth registered Dec.qtr.1864 Newington.
Also looked at original of 1851 Census for George Jacobs - his age is wrongly transcribed as 33 but is clearly shown as 22. He was a Bricklayer at that time. Perhaps he took over his father-in-law's business by 1871!
Good Luck.
Annette