Interesting discussion this.
I have never had problems with overnighting without permission - if it's high, it's generally accepted, if it's in the woods, no one finds me - but this has always been on multi-day walks. The situation is clearly very different when wanting to stop a while and "do" bushcraft. Especially when knives and fires are involved. If I am walking and sleeping "low" then I tend to cook and eat in a different place to where I camp so a small fire in my bushbuddy is never noticed (this is a "stealth camping" tip from Ray Jardine's "Beyond Backpacking" book - the ultralighter's bible but with lots of good stuff for anyone in the outdoors.)
I often spend the day in my local woods (Cotswolds or Forest of Dean) and find I can do all sorts of bushcrafty things short of lighting a fire and cutting down someone else's trees - tracking, foraging, plant/tree recognition, navigation etc.
That said, I would love to find somewhere where I can set up shop for a few days and, frinstance, sit around the fire at night spoon carving or whatever. I am pretty close to Yellow Wood and might give that a go next spring. (Sadly imobile at the moment waiting for a cartilage op). I like the idea of becoming a "voluntary warden" - have to look into it.