The only thing that precludes you from attempting an approach is an Approach Ban, this means the RVR needs to be above the required minima before passing through 1000ft above airfield elevation. On 12/30 RVR data isn't available and so Converted Met Visibility (CMV) becomes the limiting factor. This requires you to multiple the reported met visibility by a certain factor depending upon the approach lighting system and whether it's day or night. The required visibility for a CAT C/D aircraft (737/A320/767) for the Non Precision Approach to 12 is 3200m and for 30 it's 5000m. All this is based on Jeppesen Plates.
Decision height for the non-precision approach to runway 12 is 790ft
Decision
Altitude is 790ft while the Decision
Height is 691ft. The cloud base was at a
height of 700ft, meaning that the cloud base was actually 9ft above the minimums.
In the case of October 13th both visibility and cloud base were within the limits...just. Even if the cloud base was below limits I don't know of an airline that would discourage pilots attempting an approach. Most crew I know of would 'give it a go' when the cloud base is below the required minimums, especially if it was Broken as you could get a gap in the clouds. But from a commercial/customers perspective, it makes sense for the airport to delay the works if they can seeing as the cloud base was right on the limit.