The number of stamps that you need to send a letter from Australia to London, will depend with the value of one stamp.
It depends entirely on the value of the stamps. In 2010, a standard letter currently requires a stamp or stamps to the value of 60c within Australia.
3 stamps.
10
3 stamps. An average stamp is $0.44 so one would need $1.32 to send a letter from London to Utah.
1 oz letter to UK is .94
One first class postage.
Nine
$1.05 = Forever stamps are worth 45 cents each. so a little more than two of these stamps
98 cents worth
One, if it is the correct USA amount; see the Universal Postal Union of 1949.
TWO US Forever stamps will suffice (USA to London post card)
You should probably go to your Postal Office, because it depends on the letter weight, what is in it, and how fast you want it to be shipped.