The Gantry ones. Wasn't aware there was only one camera each way, every gantry has the road markings. Fairly sure they are off or at least not sending tickets as I may have gone through them a few times enthusiastically. From the news about the ones near Swansea it seems they install them and just watch the results for a while before actually starting to enforce anything.
Whenever I go out of Wiltshire to an area full of fixed cameras I always think the driving becomes a lot worse as people constantly brake and speed up for the cameras.