Here's the quick answer for the #1 and #2 Blanch Jeffery-s at the end of your post.
1. in North Hill (which is in Launceston reg dist)
born North Hill, parents
Robert and Martha2. in Preston
born "Cornwall, Devon", parents
Josiah and EmilyJosiah a gardener, both parents born Cambridgeshire
Don't forget that the authoritative source for births is the GRO index, searchable at FreeBMD.
(And of course birth certificates are the only authoritative source for parents' names, etc.)
There are only one birth in Cornwall/Devon 1889-1890 for the name Blanch* Jeff* (wild cards will catch any spelling variations):
Births Mar 1890
Jeffery Blanche
Launceston reg dist
vol 5c page 29
Even expanding the search to 1887-1892, there is no other birth in Cornwall/Devon.
Here is your household in 1891 in Sutton, Surrey, Epsom reg dist:
Josiah Jeffery 37 - gardener
Emily 36
Henry 13
Josiah 10
Hezekiah 7
Blanche 1 - born Norwood Surrey
They are in the Cottage at The Arches, home of William Palmer, a merchant with several servants.
In 1891 they are living next door to the vicar, and in 1901 they have a Congregational evangelist for a neighbour.
Did someone in 1901 mishear "Norwood" as "Cornwall"?
It still isn't clear from this what birth is Blanche, but I might guess
Births Dec 1889
Jeffrey Blanch
Croydon reg dist
vol 2a page 219
Croydon doesn't actually cover Norwood, but perhaps she was born away from home or at a transitional address if the family moved a couple of times.
(The one registered in Chesterton reg dist in Mar Q 1889 is yet another one, shown in the censuses as born in Hauxton.)
The 1911 census confirms the birth information for both Blanche-s, with the Cornwall one shown as a year older:
Cornwall Blanche Jeffery with Robert and Martha:
https://familysearch.org/pal:/MM9.1.1/JQ3X-4H2Norwood Blanche Jeffrey with Josiah:
https://familysearch.org/pal:/MM9.1.1/X7B5-GGVThe people whose trees you have been looking at, if they have Blanche daughter of Josiah and Emily as being born in Cornwall, are just plain negligent.
This is often the case with people who are simply collecting names for their trees, of people they are not directly related to. They want to incorporate the names of the offspring of the siblings of the person who married their great-aunt's older brother's wife, so they just bung in whatever they find in the census without checking it, for instance.
(I learned this lesson early on when the very first person I found with names of people in my family in their tree at another site had my father's father's father's parents as Mr X and Ms Y. I was grateful for the boost, being totally new to the whole genealogy thing, and happily adopted that couple. Fortunately, I went looking for more info about the people right away, since they were my direct ancestors. Well, my grx2 grfather was indeed a Mr X, but I very quickly found that he was not married to Ms Y - they were a completely different couple who happened to marry around the same time in the same county. The person whose tree this info was in had no reason to care about accuracy -- my Mr X's daughter had married the brother of some distant relation of hers, so the nonsense in her tree did not affect her own family history at all. Took me a year to get her to remove it from her online trees.)
So, all in all, I think we have ruled out the Cornwall connection in the case of Blanche Jeffery and her father Josiah -- but only the birth certificate or a baptismal record would say for sure!