Is it:-
‘Potentially dangerous’.
Urgent remedial action required.
Very unlikely so can't be C2.
Code C3 (Improvement recommended)
This code should be used to indicate that, whilst an observed deficiency is not considered to be a source of immediate or potential danger, improvement would contribute to a significant enhancement of the safety of the electrical installation.
The point here again is "safety" it not just Improvement recommended but Improvement recommended to improve safety and volt drop is in its self unlikely to affect safely.
However where there is a large volt drop also it may mean that safely devices can't operate as not enough current will flow but this is the loop impedance rather than volt drop although both are connected.
The
BestPracticeGuide4 has some odd things. To my mind cookers, hobs, immersion heaters, hand driers and other current using equipment is tested under "Inspection and testing of in-service electrical equipment" and is not part of the EICR however the guide says wrong type of immersion heater is a C2 code where as I would consider it was outside my remit and although one may comment on it one should not code it.
The English is also bad "Absence of a quarterly test notice for any RCD
or voltage-operated earth-leakage circuit-breaker" I am sure it does not mean the absence of a quarterly test notice for a voltage-operated earth-leakage circuit-breaker it should not be in use anyway other than as a simple isolator.
But personally I would include reference to volt drop in notes but would not code it as it does not present a danger unless it is likely to cause lights to fail.