The "count the lights" method of assessing RVR is referred to as the "Human Observer Method". Provided that the airport has lights calibrated for RVR assessments, a chart is provided to equate the number of lights to each RVR value and human observers who meet eye sight and hearing standards, the resultant RVR value reported by ATC is legal and would be used in a prosecution for any breach of UK civil aviation legislation.
Frustrating as it is when shallow fog persists, which will invariably reduce the reported RVR, if the reported RVR is below your instrument approach limits, even though you can see the runway as you go around the holding pattern, don't go there unless the reported RVR is 800 metres or greater, in which case you could probably legally opt to fly a visual approach.