Good map. Some of those are just ordinary roads - or look like them!
I was thinking the same, there's one near Roborough which is close to me. I've done some cross reference and used satellite imagery. What looked like a road at first turns out to be a dirt track that seems to cross several fields. It's probably not all that accurate but not to bad to point you in the right direction or at least give a starting point.
Not sure who said it but I can see the benefit to being a member, especially if you meet some potentially angry cyclists or horse riders.
And if it goes towards a good cause to help preserve these lanes for all to enjoy then £30 isn't that much.