My favourites are
Ruthern Valley in Cornwall, we went there about 3 or 4 years ago, great location for the Camel Trail etc
One in Edale, Derbyshire. I think it was called highfield campsite, great location for walking up Kinder Scout and Mam Tor.
and we used to camp at a site at the foot of Langdale Pikes in the lakes, but it was a National Park one that you couldn’t book, not sure if it still is.