"By my feeble reckoning then High Voltage (>20KV) kicks in when...

"By my feeble reckoning then High Voltage (>20KV) kicks in when the load is above 30,000KVA."

HV can't "kick in". The connection is 20 kV, and it will stay 20 kV. As I stated earlier MV is classified as 6/12/20 kV, and HV is classified as 25/30/... etc. Voltage regulation equipment (tap changing transformers mainly) are controlled to ensure (to the best of their ability) that voltages remain at 6/12/20 for MV lines and 25/30... for HV lines.

Once a line is designated as a certain voltage, it is regulated to stay at that voltage.

If the GMC draws more than 30 MVA then it will be put on a higher tariff, not a higher voltage