Noongallas Farm nr Gulval, Penzance
There's one in Morvah that is just a field and a wonky toilet block- you'll see it if you go to Morvah. It was £5 pp last time I went.
South Penquite Farm on Bodmin Moor, although that's on the posh side- it even has a reception!
Lower Pennycrocker Farm above (I mean 200m above, near the cliffs!) Boscastle- it has a couple of fields with EHUs where all the campers are jammed in cheek by jowl, and a couple of basic fields that were nearly empty when I went in August. I love the current trend for everyone to think they need an EHU 😈.
Trees Mill Farm, nr Tywardreath/Fowey ("I'm out DJing at a rave tonight, so just stick a tenner through the letterbox" 😆)
Keveral Farm, nr Looe.
Cockingford Mill, Brimpts Farm, Huccaby Farm- all on Dartmoor.
The Doone Valley on Exmoor has 3 campsites strung along it- I stayed in the first one which was good and basic, then there was another similar one, then Cloud Farm which is a NT campsite and looks stunning.
Paddington Farm, Glastonbury (it's behind the Tor, so you can walk there down quiet lanes)
Steeple Leaze Farm, Kimmeridge, Dorset.
All of the above are around a tenner a night per adult (last time I was there) and allow fires.
A good way to find campsites like this (that don't always have websites) is to do a search for "Farm camping" on Google maps.