MayorQuimby - correct, but of course Cadahar was talking about the IMC which is a unique UK rating and therefore I think his question was concerned with UK airspace. You can be on an IFR flight plan in the UK without any form of instrument rating so long as you remain outside of Class A and D airspace and meet the VFR visibility and cloud separation requirements. In the UK it is common for pilots to think that because they are IFR then they are also IMC - of course this is not so.
IO540 - I agree. That was of course the real basis behind my point, whilst being aware of the limited additional nav work now in the PPL syllabus.
As I commented earlier the problem with fog, just as with cloud, is that you are reliant on some other form of navigation than visual. Of course with a moving map it is easy and I guess most PPLs without an instrument rating would get on fine if they had taught themselves how to use a moving map GPS. How well they would get on with VORs and NDBs is more debatable - maybe OK straight after their training, maybe less well a year or so on if they had stopped practising these skills.
I have always thought it strange given this that in France and elsewhere VFR on top without instrument privileges is of course legal (on a French as opposed to UK license I might add). Is this because they do more instrument navigation training? It is also true of the States and you don’t seem to hear of all that many pilots getting themselves lost (do you??). Mind you I have always thought it a risky business without instrument training to get yourself above cloud relying on a forecast clearance in the cloud at your destination to get yourself down. Moreover in the event of an engine failure whilst over cloud you will still need some instrument skills to get yourself visual and that certainly is not the time you want to be under pressure when you have enough to cope with anyway!
I read a fascinating account of a fellow in the States in a Money who suffered a total engine failure at FL140 whilst clear on top, and did a dead stick landing into an airport near the edge of his glide range descending through 4,000 feet of undercast. I might add he was instrument rated, but it would be possible in the States and France for a non instrument rated pilot to find themselves in the same situation.
PS just seen I0540s post sorry for the duplication. I would just add there is at least one occasion I can think where a non-IRed pilot might declare himself IFR that is very appropriate at this time of year - flying into the dreaded sum! Often rear viz is excellent but it can be a real struggle to pick up aircraft ahead. I see no reason not to tell ATC you are IFR and why - as I0540 says you will get a better service even if they are busy if you explain why and it also does no harm adopting IFR separation.