I believe the iPhone is sold in Australia as officially unlocked and as carrier locked with a designated carrier. If you purchase an officially unlocked iPhone in Australia (more expensive), it will work with any carrier's SIM card, but Apple's support and warranty coverage is honored in the country where the iPhone was sold officially by Apple only. If you purchase an officially unlocked iPhone in Australia followed by moving to the UK, if a problem develops that can't be resolved by any other means besides an exchange under warranty, your iPhone will not be exchanged under warranty by Apple in the UK. You would need to return to Australia for an exchange under warranty, or you would need to ship your iPhone to someone you know in Australia to handle the exchange under warranty for you followed by the person shipping the exchange to you in the UK.