LEGALLY speaking, we only need sufficient visibility on the FAR-field-length, meaning that on a 10000 metre runway, only the first 2 kilometres or so (depends on your required landing distance) must be ok, the rest can be densely fogged.
Therefore in said scenario I would have asked for RVR's along the runway.
There is always a danger in landing if you see the runway from above, but the given visibility is incredibly low. From above, the fog-layer could be only 30ft thick and appear non-existent, then when the nosegear hits the runway, suddenly you try looking through 12000ft of fog.
P77