Being old-fashioned and liking paper maps, I almost always have the excellent BVA/ADFC maps on hand. With a few exceptions, whenever I've gone to a campsite marked on those maps, it's been there and happy to accommodate tents. The few exceptions have been sites that have closed down since the map was researched.
I guess if you're using other paper-based maps to find your campsite, it may be worth a quick cross check with Archies list (date handily imported into cycle.travel), or https://opencampingmap.org