I mentioned in Kevin Bradshaw's thread (least memorable weather) that fog seemed once to be much more of a feature making years memorable. The Clean Air Act has obviously improved things, but I still think that the map below shows a more foggy country than in recent years. Anyway, here are the data to back up my claim.

Numbers in circles - days of fog, annual averages over 20-28 years to 1960 depending on station. The number in the black circle (Derby or Nottingham?) is 76.7!

Outer quadrant - days of thick fog. Numbers may not be easily visible even if you zoom in, but a complete outer circle corresponds to 30 days.

Facinating stuff there David.  The worst thickest fog I ever experienced was in the autumn (October or November) of 1987 (IIRC )in Nottingham.  It was pretty much a pea souper, and I managed to get lost in the maze of grey-sodium orange heavily diffused street lights.

Foggy conditions in the late 50's were pretty dire events, and when combined with smoke (smog) caused  breathing problems for many. These smogs could last a few days at a time, and were fairly frequent, but as stated in the OP, the Clean Air Act made a huge difference.

Thick fog is quite rare in my location these days.