Swissair allowed an approach below published minimum if there was doubt about the accuracy of the RVR.(captains’ decision).
I watched a successful approach into Gatwick with a RVR below cat 2.
Monitored approach with a continued phase at minimum before the captain assumed control.
I was RHS on what was the last departure out of Heathrow before it closed. The fog layer was just reaching the heads of the transmissometers but was way below the Trident flight deck. We had gin clear viz.