Well its going to work a bit differently then reported.
First off this is for NEW customers. When you sign up you will be offered to filter adult content. Its not a default on system. Its an option that will be presented to you when you sign up.
Existing customers will not be "opted in" automatically. But they will (at least BT saying they will, guessing the other will follow) offer a method to opt in if you wish.
And remember Talk-Talk offer this service already. and Vermin Media have "OpenDNS" style DNS system inplace already inorder to get some ad cash from mistyped domains, so it won't take much to offer these services.
I agree with a network level filter even if its a service like OpenDNS and a config change on your router (if its open enough to do so) to send DNS requests though its own settings not the device's choices. Some people want to keep there kids away from porn or other content they don't want there kids to see. But it shouldn't be a UK wide opt out system.
From what I've read and watched its not the end of porn as we know it, but a method to better educate subscribers about the options they have available to them
Its not the end of porn as we know it...