Re: The 48V is needed in case of emergency
You begin by commenting "when you lose power to your house, you can't use internet anyway", then admit you have your gear on a UPS and therefore you yourself are an example of where this is not the case.
I have my modem and one of my wireless access points on UPS power and my main computers and home servers are all laptops with internal batteries. I haven't actually had a power cut since getting VDSL so have no idea what would happen to my internet connection in an outage, but in theory if BT are doing their job properly and putting batteries in the cabinets I should stay up. My main systems and hosting infrastructure is tested to remain fully functional for at least 4 hours, and things will degrade to 1 access point with emergency internet access for the next 48 hours - although I doubt they have 48 hours worth of battery in the cabinet.