I wonder how my grandkids would go. I am British by decent (my father was born in the UK), and my wife is British through birth.
We registered our daughter at the British High Commission in Canberra (sat her on the counter) when she was a baby, and her husband is a true blue Aussie, who's rarely been out of the country since birth.
If they're anything like their parents, grandmother and uncle, going to the UK would be the last thing on my grandaughters' minds. And who could blame them?
As for me, I always carry 2 passports, which I found to be useful when I lost my British passport en route from the UK to Turkey.