That's a seriously useful article, IO540 - thanks for pointing it out. (The author has an interesting website:
gCAP and publishes CAT A approach plates for single-pilot IFR).
Two very short extracts from the PPL/IR article:
Minimum RVR single pilot: The minimum RVR for single-pilot ops is 800m unless the aircraft is coupled to an ILS.
Approach ban: In fact, you can start an approach with the RVR as low as zero, but you can’t continue beyond the outer marker (or equivalent position) if it’s below limits at that time. There are hardly any IAPs in the UK now which use markers, so you need to know that the ‘equivalent position’ is 1000ft above the aerodrome. If the RVR/visibility is good enough as you pass this equivalent position then you can continue, and even if it subsequently falls you can continue all the way to the missed approach point. The bottom line is that the RVR/visibility must be at least equal to the minimum for the approach as you pass the equivalent position; before this point it doesn’t matter, and after this point it doesn’t matter.