I was flying in company with Mono on the trip in question.
We flew Popham to Kemble via South Marston and Blakehill Farm. Fairford is NOTAMmed active so we spoke to Brize en-route.
All clear of fog to South Marston, which was clearly visible. Blakehill Farm was under fog but Kemble was visible, as was Aston Down. The odd patch of ground was also visible in between and we were at 2500 ft. Effectively there was a ribbon of fog with totally clear vis on either side of it.
That's the background. From a pure survivability point of view an engine failure would have entailed heading for the nearest reachable bit of visible ground and I don't think we were at any time out of reach of visible surface.
Fog is less than a couple of wingspans deep and goes down to the ground. The outcome of an enforced descent through it is likely to be the same whether you have an IMC, IR or vanilla PPL.
So to me flying over a band of fog where you can see both sides is a bit like flying over a stretch of hostile terrain where your choices are limited if the whirly thing at the front stops going round. All aircraft engines recognise these situations and give you subtle aural stimuli just to keep you on your toes.