Back to the original post, the question of patchy shallow fog should be considered on a practical basis.
I`ve landed at LGW on 26R with reported RVRs below minima for CAT 1 ILS to 26L. On landing the fog was very low and very patchy, had 26L been in use LVP`s would have been in force.
Why ban approaches in such a scenario just because the auto RVR meter indicates a low value! why can`t the tower controller be given some discretion.
In the USA the rules are even tighter, you may continue passed the marker but if at decision height the RVR is below limits you must go around what`s happened to common sense here?
Fog is often patchy and the best person to decide if a landing can be safely achieved is the Captain at decision height.
( Airbus may beg to differ)