At the very least, the section between the car park, and the Stag Lodge exit IS one way (check google street view; there are no entry signs by the Stag Lodge gate). I can't recall off the top of my head if there are no entry signs on the western access road prohibiting exit towards cot hill, but at the very least the road layout discourages it, and the no entry at Stag Lodge creates a de-facto one way system.