i believe it is only a uk requirement!
although i seem to remember that vfr on top is considered an ifr flight by the faa for example.
in france i have reguarly experienced a milk soup kind of haze where technically you are oke for vfr .. but really ... it is dangerous.
it is THE main reason why i am installing tas in my ac.
i have to add to this that if you are on the ground it seems like a supernice day!!