You 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. You don not need to take the cloud base into account.
Where RVR is not available, the pilot-in-command may derive an RVR value by converting the reported visibility for non-precision and Category I approaches only. Use your table for this e.g reported visibility of 300m will give a converted RVR value of 600m IF the airport has high intensity approach lights AND it is night. The reason being that the lights will shine through the cloud better in the dark.
Although most airports with HIAL's will most likely have transmissometers as well.
If, after passing the outer marker or equivalent position in accordance with the above, the reported RVR/Visibility falls below the applicable minimum, the pilot-in-command may continue the approach to DA/H or MDA/H.
Where no outer marker or equivalent position exists, the pilot-in- command shall make the decision to continue or abandon the approach before descending below 1000 ft above the aerodrome on the final approach segment.
The basics are outlined above but beware of the 'applicable' minima discussed. You may have higher regulatory or company imposed limits that are higher than the Jep minima. If you are single pilot for example you will be restricted to 800m RVR.