I have been doing further thinking as to this post: http://zombiewargame.blogspot.com/2011/06/disease-control.html and I think I will set my game in the U.S. simply because of the huge number of weapons available for survivors to find.
Both the U.K. and Australia have strict gun laws and whilst there are firearms around, they are not as abundant as in the U.S. - something that isn't cool when you need a truck load of guns to fight off zombie hoards.
This will allow me to use the new Eureka bio suited U.S. figures as well as the Marine recon figures that I have been eyeing off for a long time and haven't be able to justify getting them.
I think I will also buy a FFL figure or two from Eureka and have their story as they were in the country as part of a multi-national training exercise when the apocalypse started.
Having said that - I might take the advice of those that made comments on the initial post - set the games anywhere in the world as part of a larger campaign and play different settings as the zombies spread around the world.
I most likely would use Australian and U.K. games for more of a military driven game simply because of the firearm laws.