Don't let it be an excuse to raise prices
"I would suspect these days most households have at least 3 mobile phones capable of making emergency calls."
"But can they do that in an area without a signal, or if the cell towers are down?"
The "you can't call in an area without a signal" is an ad hominem, because you're comparing to a phone that only works within like 20 feet of the wall plate in your home. Do you have signal in your home? Yes? Then, the cell phone works in a superset of the area your wired phone does. Does it work when cell towers are down? No, and the landline doesn't work when your landline is down either.
"Similarly, power outages were common in the 1970's, but are extraordinarily rare today."
"Power outages in the UK are still quite common in more rural areas, (where there is less likely to be a mobile signal) and are likely to get more common and widespread if the government policies on power generation continue."
The "Power outages are uncommon" argument is silly. Nevertheless, power outages shouldn't be an argument against cell phones either. This shouldn't affect your cell phone (not saying that it doesn't but it shouldn't.) Verizon Wireless, for instance, has battery backup and generator backup on their cell sites. If your service goes out with the power, it means your phone provider are being cheapy-cheapy.
As for BT -- I'd say if they wish to change how they provide POTS, they should be able to go ahead. But there should be a few conditions.
First, since they say this'll save them money, they should not be able to use this as an excuse to raise prices. Side bar on this topic -- when cellular phone companies in the US started adding 4G LTE and VoLTE (Voice over LTE), MetroPCS said "LTE and VoLTE has cut our cost per voice minute and per byte by more than half, we're lowering rates". Almost every other provider said "LTE and VoLTE cost a lot to roll out, we're jacking up your rates".
Second, they should still be obligated to charge the same or lower than current POTS rates, to provide a dialtone to the customer's phone jack. If they want to do it over an internet connection with a VOIP to landline phone adapter, by all means. But they should NOT be able to use this as an excuse to rope these people into paying the the same voice rate as now, PLUS an internet connection fee PLUS a VOIP adapter rental fee.
Finally, something should be done about data caps -- either all VOIP use should count against whatever cap, or all VOIP use shouldn't. I can just see BT using "our VOIP doesn't count against your cap, theirs does" as some kind of bludgeon to "persuade" people to buy BT's VOIP service. (Honestly VOIP doesn't use much data... but still.)