First make sure your phone is compatible with European networks (has to be able to work on the same bands... I don't recall what Aussie phones work on). Second, I would recommend getting a TMobile SIM card in the UK as that seems to be your first stop. You can top up at many locations in the UK or online once outside of UK, just make sure to register online once you get the card. The rates outside of the UK are standard EU roaming rates (~€0,40/min).