They should be taking into account the cable installation conditions, including grouping. However, do note that they may not be using BS7671 Appx 4 to do this*. For example, ERA 69-30 or DNO's design rules.

*Which, being an informative appendix, does not necessarily mean the design doesn't conform to BS7671... And furthermore if it's the DNO side of the meter it doesn't need to anyway.

Further to that, BS7671 cable ratings tend to be very conservative in many ways. Starting with the 30C ambient which is unusually hot for the UK, and then allowing only a 40C rise, passing through the idea that the full load and highest temperature are present all the time, and that we want a near indefinite cable life.
The DNO will 'know better' from similar installations that they can rely on the maximum load not being 100% all the time, and that short flights of  a few hours overheating a year do not suddenly cause the particular cables they use  to fail.

If you want to contrast this with the ratings of cables used in cars, where brief 110C episodes for PVC are considered normal, the apparent ratings and grouping  for the same wire gauge are quite significantly higher,  and indicators can be wired in thinner than brake lights despite being the same wattage, as they are only on half the time..

Mike.

Further to that, BS7671 cable ratings tend to be very conservative in many ways. Starting with the 30C ambient which is unusually hot for the UK, and then allowing only a 40C rise, passing through the idea that the full load and highest temperature are present all the time, and that we want a near indefinite cable life.
The DNO will 'know better' from similar installations that they can rely on the maximum load not being 100% all the time, and that short flights of  a few hours overheating a year do not suddenly cause the particular cables they use  to fail.

If you want to contrast this with the ratings of cables used in cars, where brief 110C episodes for PVC are considered normal, the apparent ratings and grouping  for the same wire gauge are quite significantly higher,  and indicators can be wired in thinner than brake lights despite being the same wattage, as they are only on half the time..

Mike.