I (and those I consulted) concluded that, going back to basics, an instrument approach is complete on acquisition of the required visual references. The landing is then completed visually. All Cat1 Instrument approaches become visual approaches at some point without further approval from ATC.
There's an interesting difference in some JAR-OPS1 and ANO requirements. My
bold:
JAR OPS 1 Subpart D 1.405
Commencement and continuation of approach
(a) The commander or the pilot to whom conduct of the flight has been delegated may commence an instrument approach regardless of the reported RVR/Visibility but the approach shall not be continued beyond the outer marker, or equivalent position, if the reported RVR/visibility is less than the applicable minima.
Non-public transport aircraft – aerodrome operating minima
49(4) Without prejudice to paragraph (2) an aircraft to which this article applies when making a descent at an aerodrome to a runway in respect of which there is a notified instrument approach procedure shall not descend from a height of 1000 feet or more above the aerodrome to a height less than 1000 feet above the aerodrome if the relevant runway visual range for that runway is at the time less than the specified minimum for landing.
Public transport aircraft registered in the United Kingdom – aerodrome operating
minima
47(9) An aircraft [required to have an Ops Manual], when making a descent to an aerodrome, shall not descend from a height of 1000 feet or more above the aerodrome to a height less than 1000 feet above the aerodrome if the relevant runway visual range at the aerodrome is at the time less than the specified minimum for landing.
JAR-OPS talks about an instrument approach, and keithl's assertion that the instrument approach is complete when visual reference is acquired (or at least when the aircraft is cleared for a visual approach) may offer a loophole for the continuance of an approach where there's visual reference but a sub-minimum RVR.
The ANO is quite clear for non-PT flights. No mention of an IAP. What matters is whether or not there
exists an IAP for the runway to which an approach is made (that in itself is interestingly ambiguous but not relevant). If there is, and the RVR is below the minimum for it, you can't continue the approach belw 1000 ft.
But what to make of Art 47(9)? "Relevant RVR at the aerodrome"? "Relevant" is not defined. Is it to be interpreted as Art 49(4), or differently? If you choose to interpret it differently, I think you're on pretty thin ice. The law doesn't normally allow the commanders of PT-flights more discretion than non-PT flights.