I imagine every PPL holder who has trained in the UK will have flown in poor visibility - I would hope that most did so as part of their course, while dual. Personally I found it an extremely valuable lesson that 3K / 2 miles visibility, while it seems fine to the lay person driving along the motorway, is seriously no fun in the air. But I would strongly resist tightening the law. Not everyone flies the same types in the same airspace, nor do they have the same skills and knowledge. What might be unsafe in a fast cruiser with limited field of view, in busy airspace, might well be safe in a 50 knot microlight doing a low level circuit out of a farm strip in the wilds of East Anglia.