Agnes PRESTON is probably the mother of Nancy DICKINSON although she is not recorded in Nancy's (IGI) christening record. Thomas DICKINSON had a constant stream of children christened in Cartmel between his marriage to Agnes in 1774 and the birth of his last child Miles in 1796. This makes a second marriage during this period unlikely. The lack of mother's name is more likely due to a change in ecclesiastic clerical practice.
Another piece of corroborating evidence is the choice of names of Nancy DICKINSON & Robert KENDAL's children which follow a traditional naming scheme:
Jane = father's mother
Agnes = mother's mother (ie Agnes PRESTON)
Margaret = father's eldest sister
Robert = father's father / father
Thomas = mother's father / father's eldest brother
Nancy = mother
William = father's next oldest brother
Elizabeth = Betty, mother's eldest sister???
John = father's next oldest brother
Ann = ??
Sarah = Sally, next eldest sister
Agnes = dec'd daughter
Mary = ??
Ann and Mary are mysteries for the moment. And why no George when both families had a George?
There is no obvious christening record for Agnes. Those on IGI include:
c.22 Ded 1747 Orton, Westmorland by William and Mary PRESTON
c. 13 Jul 1755 Kendal, Westmorland by William and Agnes PRESTON
Given that none of Agnes' ten child are called William, I suspect that neither of these is correct.
Agnes had most likely died by the time of the 1841 census. The death certificate ref Agnes Dickinson Ulverston Mar 1840 is not the correct one.