If you liked the Harry Potter Series:
Then in adult fiction, look into The Peter Grant Series by Ben Aaronovitch:
Why: It's got the hidden fantasy world thing going on, that certain Britain humor, it's self aware and makes the Harry Potter jokes, and there are twisted magical mysteries.
Other options to look into:
Discworld series by Terry Pratchett (if you liked the Brit humor side and the well developed world)
Neverwhere by Neil Gaiman (if you liked the portal fantasy/darkness of it)
The Magicians Trilogy by Lev Grossman (if you want more magical school stuff)