@ZipcodeUK - I think my top recommendation would be Treen Farm
Welcome to Treen farm | Treen Farm Campsite
It's close to a stunning (walkable but a bit sketchy near the bottom - ask for directions from the owners) beach, Logan Rock and the lovely beach of Porthcurno which has the Minack Theatre stuck in the cliff.
The site has a decent shop (with fresh croissants and essentials) and is about 1/2 mile from a really nice pub.
It's very laid back and facilities are fairly basic but it's worth it. There's quite a rural, farm feel to the place.
They also don't allow big motorhomes or caravans as the approach is narrow - IMO this is brilliant!
Downsides - it's quite hard to get to and a long way down the county.
Another favourite is Gwithian Farm next to Gwithian beach - the massive beach is walkable but I'd probably drive.
There's a good shop, a bath for little kids and a pub right outside the entrance to the site.
Gwithian is three miles long and has a great cafe in the middle (Sunset Cafe) and at the North end (an NT one - also a pub behind it).
For Devon, I really like the Salcombe area and there's two good sites there - happy to find links if needed but I think they're the only sites in the area!