Hudson Bay,
I don't think you understand.
A runway may have a number of different Absolute Minima RVR values, for different approaches - Cat1 ILS, NDB, SRA etc. (In the case you mention, the Absolute Minima RVR value for a Visual Approach to RW 06R at MAN is 800 metres, for example. Should a pilot attempt a visual approach when the RVR is less than 800m, then ATC are required to WARN, using precisely worded phraseology, and if ignored, to subsequently REPORT the facts.
However, the 'ATC Absolute Minima Procedure' does
not apply to CAT II/III Approaches, as bookworm's quote from a CAA document clearly states.
Please have another look at the
LINK I posted earlier.