That’s true and it would be much better but we still have more rights than many people would tell you.
E.g. you have the right to “rest” on land adjacent to the side of a footpath or bridalway. Now the definition of “rest” is undefined because that’s how common law works.
So you can probably make the argument resting includes camping over night (when leaving no trace) until someone proves in court that it doesn’t.
Here’s another fun fact: you can also legally ride a bike on a “footpath” so long as you discount when you come to pedestrians. That one was clarified in Hansard but people blindly assume that they don’t have that right without checking.
The point is, things are usually legal unless there’s a law and legal precedent to say otherwise.
What if the trespassers just ignore the sign, and walk on/over the privately owned land without the landowner ever becoming aware of it? How could the landowner sue the unknown trespassers when there wasn't even any evidence of their trespassing?
Yes, that's what I was asking. Your previous comment indicated the homeowner needed evidence in order to sue for trespassing. But it seems there was no crime committed after all.
"Not criminal" doesn't mean "not illegal". Breach of contract is a civil matter (in general) but it's still normally described and understood as illegal.