Naked Broadband without line rental, is something that can easily be supplied, but suppliers prefer to keep charging line rental
I suspect the cost would be broadly similar.
The line rental (I pay Primus 5.99 a month) is intended to cover the link to the exchange plus the exchange to exchange network, standby generators (if available), the batteries (so your corded phone will still work during a power cut) at the exchange, and general maintenance of the cabling and buildings.
What you pay for broadband is for the network infrastructure, so
if "Naked Broadband" would somehow have to add a fee to cover the standard phone cable and network infrastructure on top of the broadband network kit. I am pretty sure that if you can get it (eg overseas) many countries charge significantly more than the basic broadband for this.