A4: <<But if you had RVR 800m......you'd just fly a CAT 1 ILS and land >> On a practical note, circling to land off a perfectly adequate IAP is normally done because some other factor precludes landing on the IAP RW - usually wind being outside FM tailwind limits - rather than playing semantics with minima. On my fleet we have a destination in India which regularly offers an ILS with a tailwind - if more than 15kts (my 'plane's TWC limit) we must fly the ILS and circle to land on the reciprocal which has no IAP.
I know your question is trying to squeeze a fine point of regulation - but often the situation is actually simpler in practice because if there's that much porridge you're probably off to your #1 div anyhow. But to bravely attempt to address what I think you're after - no: I don't think you may progress beyond or below the approach ban point or altitude without the IAP minima for the IAP RW. And you cannot circle without visual reference (or 800m RVR on the landing RW).
You may, though, fly to the ban point without the IAP RW minima being met, and I suppose if you had the visual references required and 800m RVR for the landing runway you might argue that it would legal at that point to break for the circle. But how likely is it that? I don't know, but I would opine that you're really starting to dig holes in very limiting conditions that you may reasonably decide are not worth digging....