[samstokes]

It sounds like TalkTalk is actually one of the last ISPs to do this, not the first:

In October, major internet suppliers including BT and Virgin said they have decided to change procedures to that when new customers sign up they will not be able to progress with accessing content until they decide whether to activate parental control over content. Previously the decision about whether to control adult content was made at any point after the sign up, when adult content may have already been accessed.