Going to add a tiny additional snippet, especially if someone can get hold of a Southwick marriage and burial register.
As Andrea found the only Southwick marriage of a John Upton was to Rhoda Sturt of New Shoreham in 1828, what if this was a second marriage? It may therefore be worth having a look to see if that John Upton was a widower and if so, for the death of a Virtue Upton between 1818 and 1828.
If that was not the case, you could try to find out if the couple (John & Virtue) survived long enough to be enumerated in the 1841 & 1851 censuses. Richard Adams Upton, the son who married in 1845, might even be found living with them in 1841, or alternatively with his father and step mother.
I know much of this is still speculation, but when you are stumped, it occasionally helps to think a little bit outside the box
Roy G