It's better to have more information than not enough. Hey if they've got a problem with you they can put you on the next plane back to Oz. These can be obtained from the British High Commission in Canberra . Here is a link which details fare information from each London airport into zone 1 in the city. This is popular phone card brand which is available from Off Licenses and some newsagents.
Phone cards are definitely the way to go when calling home from within the United Kingdom.
As an example a 5 card will get you around 120 minutes of talk time to Australia. After you've arrived there are a number of things that need to be sorted out pretty quickly. Accomodation Like a place to live. Don't fall into the trap of giving an estate agent a house finding fee. I think your better off buying the loot which is like the Trading Post from home.