There are a number of issues here.
A "Low visibility take-off" is a take-off on a runway where the RVR is les than 400m
So when the RVR is at least 150m but less than 400m, LVPs must be in force (there are certain rules for non-CAT 3 airfields).
In this case, the reported RVR/VIS of the initial part of the take-off run can be replaced by pilot assessment. Thus if the reported RVRs are 100m/150m/150m and required RVR is 150m then provided the pilot when lined up can see the required 150m, the requirements have been met.
The midpoint and stop-end RVRs can not be replaced by pilot assessment because if you could see that far from the threshold then LVPs would not be in force!
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Subject to approval from the authority the minimum RVR can be reduced to 125m for CAT A,B and C.
In this case the requirements were as described by Right Way Up.
The required RVR value must be achieved for all the relevant RVR reporting points ( no substitution allowed); and
A 90m visual segment must be available from the cockpit at the start of the take-off run - This means that with RVRs of 125/125/125, when lined up the pilot must be able to see at least 6 runway centerline lights (15m spacing) before commencing take-off.
I believe that the idea is to prevent take-off where for example the fog is thicker on parts of the runway than at the RVR sensors or to allow for the fact that the B747 pilot may be up in the very low cloud while the citation pilot is below and can see further.
I remember Gatwick's RVRs sensors being very close to the ground and reporting very low RVRs when the fog only reached half way up the aircraft legs!!
Regards,
DFC