And surely on a 3rd other hand it involved an awful lot less work than rolling out FTTP.
Yup
It meant that they could leave a lot of the FTTP work to be done as and when individual customers opted to take a full-fibre broadband service, either from BT or a 3rd-party provider,
Afaict there are four parts to an openreach FTTP connection, exchange to aggreation node, aggregation node to splitter, splitter to DP and DP to customer.
Exchange to aggregation node was built out as part of the FTTC rollout.
Aggregation node to splitter node to DP is normally built out on BT's initiative, before they start taking regular FTTP orders. There is a process called "fiber on demand" where you can pay them to build out but it's clearly intended as a special case.
DP to customer is only built out when someone actually orders a FTTP service.
BT has a broadband market share of under 30% - maybe even nearer to 25%. They know how many of their phone customers have BT broadband, what they don't know is how many of the others have no broadband, or have non-BT broadband.
I would expect that when a line carries both phone and broadband services, that information would be visible to both the phone and the broadband provider. Whether the phone and broadband provider can see exactly who is responsible for the other service I don't know.
What they won't know about is people who have broadband that is not delivered over their phone line.
I don't know how typical/atypical I am, but prior to becoming a BT broadband customer I'd had it from a number of other providers, but I'd always had a BT phone landline.
Yup, that was certainly the case historically. My parents got their first broadband connection from Pipex on their BT phone line. Then when Pipex was bought by tiscali and turned to **** they moved to IDNET, all while keeping phone service with BT.
But then when they called up IDNET to switch from ADSL to FTTC they were sold a bundle deal.