SOU
SOU is susceptible to fog simply because of its location in the Hampshire Basin. It is almost at sea level (02 threshold 33 feet AMSL), and it has the River Itchen running alongside it. This morning the RVR was 125 meters.
Investment will not fix this.
As for the point about airlines not being able to run the sunshine routes with larger equipment, as I have said before A319 and A320 aircraft are entirely suitable for operation out of SOU to Spain and Majorca, as they have proven in the past.