AFAICS the situation is this:
For reasons of promoting competition BT was shut out of cable provision for years. When it became clear that the competitive situation wasn't going to deliver anything like a nationwide service once the cherry-picking was done BT was allowed in and started the much bigger investment of building a much wider FTTC network. Being an experienced telecoms company they laid capacity for expansion; much of the cost is in all the field operations so including the spare capacity now is a relatively small investment compared to what it would cost to do it later.
Now everyone who didn't make such investments in the past and don't want to do so now or in the future want to be able to leech off BT's investment. And if that happens then at some future point when BT needs the capacity that they laid but no longer have and thus fails to provide some service whose fault will it be? BT's!
Biting the hand that feeds IT
