Isn't this (pretty stupid) situation impacting takeup of the FTTP service?
The basic problem is that broadband products are such a low price now, that the only way to realistically make any money is to have shedloads of subscribers.
Any retail ISP has to design a retail-level product that incorporates the wholesale FTTP products, and sell it at the right price, with the right package of allowances. They have to put together the package on the sales website, and incorporate it into support and staff training. Their back-office systems have to integrate with BT Wholesale to allow ordering and trouble-shooting. All that takes time and effort... so they have to see some volume of customers to make it worthwhile.
The downside to FTTP customers is that they can download a hell of a lot more than the average FTTC customer, so can fundamentally break the ISP's budget for core network costs.
Finally, there are relatively few FTTP subscribers beyond the original trial areas, and so the even smaller fraction that choose anything other than BT Retail's Infinity may not be enough to worry about.
So more setup costs, and more ongoing monthly costs, but not enough customers to chase.
It certainly affects choice, but does this affect take-up? The service is available from BT Retail, and the majority of fibre subscribers choose Infinity anyway.
What's the solution?
Increase the volume of subscribers whose area is provisioned with FTTP significantly.
Or cause the nation to suddenly realise that paying more for broadband is a good idea, to allow future investment to go ahead. A mass epiphany...