I can buy broadband from at least a dozen ISPs. But they all use BT Openreach's cables and exchange, most use their network too, and they all pay BT whatever wholesale cost they charge them.
Competition is largely pretend.
The only real competition is with Virgin, mobile and the newer fibre companies. Hopefully competition will increase, a minimum £300 a year for a network connection seems like a lot of money for not very much to me. Hopefully this will fall after all the current investment in fibre is over and done with, there shouldn't be a need for an upgrade for a good while after everyone has gigabit each way.
I have a gigabit connection to the other end of my bungalow that's longer than many people's phone lines. I know how much it cost to install, it was about £30 upfront with absolutely no ongoing costs or maintenance. I'd be very glad to rent it out for £25 a month, seems like a good scam to me.