It may have already been said before, but although you maybe required, by law to remain 'clear of cloud' and in sight of the surface. You have to way up the pro's and cons. Do you descend into the fog to try and find, the surface? Hit the surface or obstacle? Or remain VMC, assess whether the fog is due to local topography (valleys etc) and continue on same track. Or Turn back to where there was no fog, and land safely.
I would rather survive and be bollocked for bending the rules, than be injured or killed for trying to apply the rules.
I have only a little flying experience, so that is only my opinion, and the opinion of some others spoken to on the topic.
But with all that in mind, son't forget to ask your friendly local ATC unit, or even D&D (in UK) for help, if you ever get into difficulty.
Safe flying!!!!!!!
