I can recommend the cool campings book, I did a tour of the west country and ended up in some very cool and small sites. I got the Europe version too but the sites while cool to tend to be bigger.
I can personally recommend: http://treenfarmcampsite.com/ Near Lands End.
Pro tip: You cannot book this site. BUT in Treen there is a field for parking in, the old boy who runs it allows overnight stays for £3, and there is a great pub about 100m away.. So you can be first in the Q the next morning to get into the site.
Its also home to the FirstNLast VW club annual show, well worth a visit. Not many VW shows you can also go surfing at.
www.fnlvw.co.uk Party on the Friday and Saturday nights, show n shine on the Sunday.
Mat