BT Copper, going, going ......gone.

The problem is not the fibre optics-  personally I'd be all in favour of that for EMC reasons (and as a young researcher with side burns I did a lot of work in my early 20s on the stuff. It was new and strange back then and no-one was sure how it would be used, but we knew it would be important one day.)
The problem is the lack of joined up thinking about power cuts, and the absence of a requirement for mobile phone masts or home units to have any significant  back-up battery - and it is not done because it is a hard ask, more than a few AA cells -   when idle a typical home unit consumes an amp to an amp and a half at 12V, (about the same as 18 television sets on standby),  - so 48hr back up for that needs something like a fully charged car battery in each house !! 
A cell phone base station is more like a kilowatt, so the batteries required to survive a power cut much more than an hour or so get impractical in a box at the roadside.

Actually it is not just BT who have not considered this properly - it seems the Russians invading Ukraine cut power to many mobile phone base stations as they went in, and then found that their 'ERA' designed secure links back to home that were designed to piggyback onto local 3g/4g phone networks did not work after a short while. so they fell back on the VHF walkei talkies and those can be traced, intercepted and jammed very easily.

Mike

The problem is not the fibre optics-  personally I'd be all in favour of that for EMC reasons (and as a young researcher with side burns I did a lot of work in my early 20s on the stuff. It was new and strange back then and no-one was sure how it would be used, but we knew it would be important one day.)
The problem is the lack of joined up thinking about power cuts, and the absence of a requirement for mobile phone masts or home units to have any significant  back-up battery - and it is not done because it is a hard ask, more than a few AA cells -   when idle a typical home unit consumes an amp to an amp and a half at 12V, (about the same as 18 television sets on standby),  - so 48hr back up for that needs something like a fully charged car battery in each house !! 
A cell phone base station is more like a kilowatt, so the batteries required to survive a power cut much more than an hour or so get impractical in a box at the roadside.

Actually it is not just BT who have not considered this properly - it seems the Russians invading Ukraine cut power to many mobile phone base stations as they went in, and then found that their 'ERA' designed secure links back to home that were designed to piggyback onto local 3g/4g phone networks did not work after a short while. so they fell back on the VHF walkei talkies and those can be traced, intercepted and jammed very easily.

Mike

Quite.

Roger Kemp, who led the team which designed the first Eurostar trains at Alsthom (as it was then), wrote up what happened in Lancaster in December 2015 https://rvs-bi.de/publications/Reports/KempLancasterPowerCuts201512V3.pdf  He also ran an RAEng seminar on it a few months later https://www.raeng.org.uk/publications/reports/living-without-electricity  Natasha Bernal at Wired has a good write-up on BT's plans from September 2021 https://www.wired.co.uk/article/landline-phone-network-death but fails to mention the issue of communications during blackout.