That’s sucks. I’m not from the UK but isn’t there already a line rental fee she has to pay monthly or yearly? Is the “upgrade” more expensive overall?

Seems maybe a mobile phone is a better solution. At least they still work if the power is out (not if it kills the cell network too of course).

But your mum would have to change her phone number and learn a new device. And remember to charge it.

I remember getting my grandma a mobile phone for emergencies (Nokia 3310) and it was so hard for her to grasp how to use it.