The whole Unimog/tractor thing is a mare! What bugs me is that however you register you get limited one way or another. If you use red you can't haul goods. If you register as a lorry you can't have traces of red in the tank. Even if you've been using it as an off road power unit or a chipper or to haul timber from a private wood you're limited to white. It's certainly a tough one.
Can you register as a tractor but run on white if hauling?