If it is an unadopted highway, that only means that it is privately maintained, but it may or may not be subject to statutory control. Check for any Traffic Regulation Orders (TRO) or other regulations governing the road, as these would indicate statutory control. If such controls exist, the road would not qualify as "relevant land" under PoFA. If no such controls are in place, the road should be considered private land under PoFA, and keeper liability could apply.