I was on the same event as Kim, and I don't actually remember riding in anything like that in decades. Even then it was only foggy on the tops, as Kim says - down in the flatlands, on the way from Aylesbury, it had all cleared. As I mentioned elsewhere, the sneaky trick I used was accidentally to turn up one hour later than I should have done for the start, thus allowing the fog to clear more by the time I went through, but it was still thick around Christmas Common, and I was making some of the same observations (with basically the same conclusions) as did Kim.
It did remind me of riding home, maybe the first or second year I was at senior school. That one was so thick that my Mum came out walking, looking for me. As I remember it now, I couldn't even see her on the pavement next to me. Youthful innocence, but I felt perfectly safe because the cars basically couldn't move at any speed, as they couldn't even tell where the road was.