Hello...
as you already know...the CAT 1 minima (MDA) is always 200 feet above the landing thresh-hold.
and the CAT 2 minima (DH) is always 100 based on radio altitude.
so if you commence the approach to cat 1 minima and having known that the cloud is overcast at 100feet...there is a big big chance that you might not see the runway.
legal or not....my insturctor told me once...if you will use precesion approach ..you can commence the approach if you have at least the required RVR.
if you are Non precesion approach...you should have cloud base not below your minimum decent altitude.
but this what my instructor told ...please don't judge on it.