I understand what you're trying to achieve.
What you need to do is group widgets a certain way. Instead of having ALL your widgets be children of UIGrid, you need to group with into "pages". For example, first "page" is a game object underneath your UIGrid, called "Page 1". Its children are 1, 2, 3, 4, 5, 6, 7, 8, 9. Second page, game object called "Page 2", and its children are 10, 11, 12, 13, 14, 15, 16, 17, 18.
Each "Page" needs to have its position be directly in the middle of the group of widgets.