I have worked in fog so thick that the driver couldn't see the ballast from his cab window. It didn't stop the train but movement was at less than walking pace. I was flagging at the time and didn't see the train creeping toward me until it was hardly more than arm's length away. Frightening conditions. That wasn't even a freezing fog.
Back in the 70s a freezing fog blanketed London for several days and nearly everything stopped dead. On LUL they had Fog Repeater Signals and even those didn't help much. Both ends of platforms were marked with some sort of flare arrangement - I suspect they were buckets of rags soaked in oil (or similar).
BR "Fog Working" is a whole subject in itself. Essentially, with Absolute Block and its derivatives, "Fog or Falling Snow" becomes the controlling condition when the Signalman cannot see his Fog Marking Point. This would normally be a specified location at 200 yards from the Box. Once the FMP vanishes Double Blocking would be introduced until Fog Signalmen were in position to "Fog" the nominated signals (primarily the Distant Signals - except at junctions where it got more elaborate). Fogmen weren't required where the Distant was a Colour Light and CL also meant that Double Blocking wasn't required.
I hope this helps.