The reasons is that both countries offer very good rates with local SIM cards. I would recommend the Lebara SIMs for both the UK and Australia. For both countries, you get calls back to the US for about $0.05 per minute (besides the free incoming calls). Also, most people who are going to Australia are there for at least a week. If you were going to Australia for a couple of days and then a couple of days in the UK, then a roaming SIM would have done the trick. The Lebara SIMs are not easy to find, especially in Australia. If you just pick up a local SIM when you arrive (Optus for example), the rates will not be close to a $0.05. You can get the SIMs online before you leave from Amazon (less expensive but zero customer service) or Cellular Abroad (costs a bit more but they throw in shipping plus they have excellent customer service -
www.cellularabroad.com)