I think you're in danger of validating the CAAs current efforts to codify good airmanship with a stock visibility limit. To avoid some justified eye rolling, can I suggest your question might be better framed as "how should I fly in conditions of deteriorating visibility" ?
A technique which made sense to me was always to try and keep your forward reference point (ie your apparent horizon) the same distance away from you. If it starts coming towards you, fly towards the point you can see (as opposed to straight and level) and this will maintain your visual reference and cause you to descend. Each time you're forced to do this you slow down and when your judgement tells you it's time to stop, you make a precautionary landing.
If you have no forward reference point, you've already blown it.
Hope that helps.