I believe that in both cases the official camp sites are for full season ticket holders only

According to this

http://www.sidmouthfolkweek.co.uk/index.php?page=tickets

you can buy a "campsite passport" for 35 pounds and a week's camping ticket for 60 pounds on top of that. Which is reasonable.

I don't much fancy hauling my instruments, tent, bedding and clothes for a week for 600 miles by bus, though.