I don't know how the levels are set, Ummmm, I can only go on the fact that some years ago I found myself having to claim benefit (as a single person).
Can't remember the exact figures, but I do remember that the amount I was given was meagre, but would've gone up quite considerably, had I been claiming for a dependant.
OH used to work with a chap with three kids. One day he told us how much extra he and his missus got (tax credits of some description) for the kids. I nearly fell through the floor. This man had a massive grin on his face, and I could understand why.
This gave me the impression that if you have dependants, you get a lot of help.