Obviously the issue for Krezack is that our seasons are reversed. Personally, because our climate is so wet, I'd plan on late May into early June or late August into early September. You might incur high shoulder fares late summer though because of the school holidays.
London needs a week at least. Plan carefully and you might even get half decent accommodation. The majority of the important galleries and museums are free. As for useful int, well London is full of Antipodeans with their own extremely complex and extensive survival network. If you come to London I will buy you a beer.
I am biased, but you would be a fool to miss out on Italy. Paris? Meh. Florence, Sienna, Rome? Yes.
A September trip would of course allow you to travel to Italy via Munich for that feral Aussie rite of passage, the Oktoberfest. I am going next year, I might see you there.