I also suspect the relative ease of relocating a large number Australian's to the UK would have a big impact (ease of getting visa's etc).
Lots of Australians would qualify for permanent residency in the UK under an Ancestory Visa, or an equivalent in another EU country. Emigrating AU<->UK is a very well worn path that's easy to naviagte. The US isn't quite so welcoming.
(Source: i'm an Australian who live in the UK for 10 years, the US for 2 and now Canada for 5.)