I'm sure it was rubbish ! How on earth would the rest of the passengers know if the crew were signed-off or not ?!
I think Ryanair's 737s are CAT II and it may have been the conditions at Stansted were worse than that or the aircraft may have been carrying a defect that rendered it CAT I. Or... the ILS at Stansted had been downgraded, the localiser might have failed, the approach lighting might have been out... who knows !
How do you define fog..... I used to know this... it's to do with the size of the water droplets suspended in the air believe it or not ! At some diameter of droplet mist becomes fog... or is it the other way round. Ahhh... meteorology. Purely a definition, and the real-world application is how far you can see in a straight line through it.