I don't believe there is any significant difference between the US and Australia and the UK. There have been quite a few surveys, but they have differed in methodology, and most have either taken people's word for it, or made it likely that smaller guys are less likely to take part, which makes their results useless. When you take those out, most (but not all) of the good surveys find an average erect length, non-bonepressed, below 5.5" - usually below 5.25". However, there is still considerable variation.
There are all sorts of anecdotal stories - that the French are small, the Scandinavians large, etc etc - but no real evidence to support them. Italians are often said to be large, but an Italian survey a couple of years ago found an average close to 5.1".