Perma - its a good question! Working from JAR-OPS 1.430,App 1 Table 8 we have:
Lowest met vis for circling (Cat C) 2400m which I have always understood to be 'in-flight vis'
Minimum
RVR (my bold) for a 'visual approach' 800m.
My reading is that I can circle with 2400m
MET VIS (should I be so bold
) but could not do so if the
RVR was <800m. It is a matter of interpretation I guess and not an area in which I would willingly be 'grobbling about'. I take your point about JAR-OPS's wording regarding 'straight-in' approaches.
EG shallow fog bank on the runway, 10k+ vis above in the 'circuit', runway in sight all the way round the 'circuit' but 700m RVR, no circling or 'visual approach'.
That's the way I see it but I'm sure someone knows real answer.