Hi Ollie Onion,
Regardless, at ANY point on the ILS, if you sight the runway and make an assessment that you have the required VIS then surely you can continue based on this, are you not licenced to make this assessment.
Suppose the fog is just 50 feet deep.
From a point far out, you may be able see the runway quite clearly through shallow fog, because the light only has to travel through the fog a distance of 50/TAN (3degs) = 953 feet (or about 320 m). So if the RVR is 350m, then you'll be able to see all the runway easily.
During the flare, you'll be looking along parallel to the runway and will only be able to see 350m.
That's why we are licenced - to pay due regard to the reported RVRs.