Trippet stones on Bodmin moor is do-able. Close to the road, slightly uneven in places. I'm fairly sure you could get a chair there. Long meg is no problem.

My daughter uses a wheelchiar but only a little one, we're looking at off road types of chair, but aren't getting much better than oversized 3 wheeler buggies.

I remeber a while back seeing something about legged, spider-type contraptions but they were prototypes. Don't think anything came of it, and I imagine the martgage needed to buy one would rule it out for most.