Maybe UKPN should attend the next PAC meeting to explain their variable costs.
That would be an interesting discussion.
I thought the answer, though, was roughly the same as for the financial problems of rural broadband: Connecting to power used to have a similar cost for any line, with the company using the knowledge of long-term future income to offset any major incremental costs for one line.
However, the appearance of competition in the market meant reduced prices as "standard", but no more long-term offset: Any line that costs more than "standard" must pay more than standard, a lot more.
It seems that regulation of mature markets like this, imposing competitive tendencies, has a serious knock-on effect. A law of unintended consequences.