I can tell you that the US intonation is also often heard now among British teenagers: the dominant tone, even in my part of the south-west, seems to be half-Basildon (a new town in south-east England: google "Estuary English" for the full context), half-Southern California.

I do get the impression that the main trend in Aus/NZ/Canada in recent decades has been a breaking of ties with the UK and its replacement by the US as a cultural model for the new generations. I get the feeling that most young people in those countries probably now see the UK as a played-out fossil nation and, if they're of British descent, can't understand why their parents and grandparents still relate to it.