Aceostrat
If you don't have the required RVR's you cannot continue the approach assuming you are above your company "approach ban" altitude. My company uses 1000', so if RVR's not correct at 1000' GA. However, if the RVR's were good at 1000' but subsequently went below minima when A/C below 1000', we would continue to minima and if the required visual reference was available Land, if not GA.