Hello Tom ... I also live in Australia ... I book a cruises every year with a US Agent to get a better deal than here in oz.
Princess are great but I found I can not book them via the US as I live in Australia. I saw another post from UK were a person had the same problem for a cruise from the US. Guess it's because they have an office here in Australia.
If booking with an overseas Agent I would make sure they know you live in Australia and it's ok with Princess Cruises.
I always book about a year before to get the cabin I want .. it also gives me something to look forward to.
UK and a Cruise sounds the perfect holiday.