I would say everyone will have a very different answer to this - I mean where do you start as you're talking about 300 million people. If you live in Aus the plus point for the UK is that the culture shock will be very minimal compared to the rest of the EU. There are parts in London where you will be in a majority.

However, this might not be what you're looking for.

Take a country like Holland where they speak English better than us Brits. Great place where not speaking Dutch wont be a barrier (assuming you can't) but very different to Aus.

I guess it comes down to how much of a change you want and then go from there. If you want a massive change go to Southern Europe if you want a little bit of home then the UK makes alot of sense.