It's an offence under the Protection of Badgers Act if a person has in their possession "any dead badger or any part of, or anything derived from, a dead badger"
I'd say that covers a skull but I'm sure you'll be alright as long as you don't start waving it around in front of the RSPCA.
What if you have, in your possession, "any LIVE badger, or any part of......" ?
Is the shite protected too??