For what it's worth, I too think that Sarah's place of birth is indeed Chulmleigh, Devon (I note that in 1851 the John you mention has birthplace as Chumley).
There was only one John baptising children there at the relevant time and that's the one you've mentioned, wife Elizabeth - equally so, it's been proven that their daughter Sarah bp.1809 married Charles Kitchener in 1841 Bethnal Green (marriage certificate clearly states her father as John Amery, Builder).
At the end of the day, your only link to 'your' Sarah's father being named John is the name that was given by the informant of her death. This could have been an error, misremembered or even guessed at.
I would suggest that your Sarah was the one bp'd 26/7/1804 Chulmleigh, dau. of Richard Amery and wife Elizabeth (nee Lyle married 10/7/1796 Chulmleigh). I note that Sarah and Edwards' youngest son was named Richard, another being Philip and Sarah b.1804 had an older brother of this name.
If John Amery was actually your Sarah's uncle she may have moved to London with them (for whatever reason) and if spoken about in the family it might be why the name John was remembered by her family (assuming it was a family member who was the informant).
My instinct is that your Sarah was bp.26/7/1804 Chulmleigh, dau. of Richard and Elizabeth (there is no further trace of this Sarah marrying in the parish, or appearing on census, suggesting she left home at some point).
Annette