Like many things in life, there is the quick practical answer, and then there is the real answer...
The 80% factor is for cold-measured circuits to allow for them running hot, and is also based on a roughly 50C rise in operating temperature as typically most cables have the CCC limit based on 70C PVC and most UK places are around 20C when at room temperature.
If your measured-to-operating temperatures are different then the 80% should differ as well. So if very high temp (e.g. MICC cabling in some steelworks style factory) you might need less then 80%, or if the cable is long so has to be larger CSA for VD than the required CCC would otherwise dictate, then you might above 80%.
