Just curious as to why regularly they close the opposite carriageway for the air ambulance. If the northbound carriageway was closed because of the accident why not land there. Plus you don’t have to carry folk over the barrier.
Diversion2, the opposite c/way is closed purely to stop flying debris, caused by the downdraft, hitting vehicles and to prevent crashes caused by rubberneckers looking at the helicopter landing. Usually, the opposite c/way is blocked whilst it lands, released, then blocked again for take off. I have known London HEMS pilot to park his helicopter on hard shoulder/lane 1, so traffic can pass in lane 4 on the same c/way,whilst the chopper is on scene, which is very nice of him. 
Generally, the helicopter always lands on the same c/way as the incident.