For Cat I approaches a 'human observer' RVR can be measured. This is typically done by someone counting the number of edge lights that are visible from a particular place near the runway threshold. To do this the lights normally need to be 'calibrated' periodically - that is, the number of lights visible need to be converted into a RVR value. In the UK the CAA do this (certainly used to anyway) but it's costly and as soon as most airports go Cat II/III they get transmissometers and drop the human observer option.
I suspect that if Oxford reports RVR it will be based on human observer measurements. Indeed, I would guess that the majority of Cat I (only) airports that report RVR use the human observer method.