Might be easiest to find a course first, then (if it's not located at a campsite) find a suitable local site on Camp Site Search?
Meanwhile, check your nearest The Works bookshop for Self-Sufficiency Foraging (ISBN 978 1 84773 772 4), currently £1 in store/£2.99 (plus p&p presumably) online. Don't think it mentions camping, though!
http://monicawilde.com/woodland-foraging-walks-workshops-scotland/ - but you'll need to come north. Then, as already suggested, once you've found the course then use the UKCS to find the ideal site nearby. The forestry commission ones are usually ok. And http://forestforagers.co.uk/ has courses at the other end of the country in Essex. Sounds fun! And it's good to learn what's tasty and what might do you harm. It's sad to see children yanked away from picking brambles.
Thank you for this thread I hadn't ever thought about this, sadly I don't think I'm physically able to do any of the courses but I can send my son so he can show me :)