One of the problems SOU will always face is the lack of Cat III ILS. I think there is not enough space and too much metal around to allow a protected area.
The fog occurs as the airport is right next to the river Itchen and in a shallow valley.
When I worked there 2017-19 there were a number of occasions when all or most evening arrivals diverted. Not good for the passengers and not good for the morning departures, ie. no aircraft.