It’s not strictly speaking legally protected but it’s not illegal either.
Trespass in England is definitely not yet a crime[1].
Apart from a few specific areas like Dartmoor where wild camping is specifically legal it amounts to, you can camp where you like but you can’t damage anything doing so and if you get asked to leave you have to.
If you don’t leave when asked to by the land owner only then it becomes a criminal matter.
The whole process becomes even murkier because it’s often hard to know who actually owns the land. A random member of the public can’t ask you to leave.
It turns out that land owners don’t often want to admit they own the land.
The whole thing is a bloody mess but can be summed up as, it’s easier to beg forgiveness than ask permission so leave no trace.
1. I say not yet because several wealthy Tory party donors have campaigned for years to change the law.
https://www.cps.gov.uk/legal-guidance/trespass-and-nuisance-...
I've lived at two Bruce Trail trailheads, and thankfully, most people are responsible. The problem where I am now is that these laws and rights of way were made when the population was much smaller. We get loiterers, "kids" but really people in their 20s from nearby suburbs with subwoofers in their cars parked at the trailhead and yelling. Rock faces on the trail are covered in spray paint, and not the grafitti art people are so sentimental about.
I could really see a revolt against these old rights as societies change. They were made for a different time culture where there was a recognition that what made them beautiful and desirable was our respective absence and invisibility. "Leave no trace" is a pretty refined value and it's not universal.