I recently cycled LEJOG, and although I sort of pre-booked my sites along the route, I had no difficulty in just turning up and camping. There were two of us, and it was a charity ride, so a few places even gave us discount.
Most sites expect a family car with a huge tent on each pitch, so if you are alone in a small tent, they often charge a lot less. If you are a little cute, you can pitch up late, and leave early and avoid paying altogether. Though I would never advocate this, it happened a couple of times with us, when we literally couldn't find anyone to pay !!
We also managed to wild camp on the odd night, but obviously there are no facilities, and I slept in my damp cycling gear. I dried off the next day. If you are hiking on Dartmoor etc, you can camp wild alongside the paths if you don't mind getting a bit smelly.
If you want a list of sites, use the UK camping website
http://www.ukcampsite.co.uk/I found this an excellent resource.
Good luck
If you need more info, drop me a private message.
Regards
Paul