Actually, having flown yesterday both morning and afternoon, the visibility was variously reported as around 8-9 km. It was reasonably hazy, and I was grateful for the traffic service I had for the afternoon flight in the afternoon which was mostly in a westerly direction (towards low sun in haze) because spotting stuff was hard enough even knowing where to look.
This is my biggest fear when flying.
Statistically unlikely in open FIR but the airspace designers seem to want to push us into smaller and smaller areas under the LTMA and between airports where the density of VFR traffic makes mid-airs much more likely. Personally, I route away from such areas, even if it means a substantial diversion around London. Thankfully, in poor viz, Farnborough now seem more willing to give a traffic service but this can't overcome the fact that you have to put your head down to read a map, write down a squalk and then put it into the transponder, check your heading, change tanks, change frequency........ all things that keep you from looking out.