There's always going to be a component of the cost that covers maintenance of the cable and head-end equipment. Last I heard the cost of providing a voice service was a couple of quid a month so that would suggest a minimum fee of £10pcm with the costs of providing a data service on top of that.
Right. The maintenance and overheads dwarf the cost of moving a few kilobits of voice data for a few minutes here and there.
If you live in a dense but modern area, Openreach doesn't spend anywhere near £10pcm providing service to you unless there's a disaster. And if you live in the middle of nowhere with a mile of cable on poles just to reach your phone they've probably spent more than £10pcm looking after it, costs for a guy with a cherry picker would get though a year's line rental in an hour or so. That's how it goes with universal service, some of us subsidise service for the rest, but everybody benefits from the resulting network effect.
You can get line rental that's literally just that, rental for the line but no calls bundled or whatever, and they're often a cheaper option if you really won't actually plug in a phone. A&A (who aren't exactly cheap in general) charge me £10 per month for this IIRC.