If you click on the link quinella66 posted above, you will find that on Chart 1 (which is applicable for BA) the award MIA-LHR will cost you 50000/100000/125000 miles depending on the class of service. (The direct distance is 2x 4425 miles.)

That means you'll need 100k Qantas miles to get that free biz class ticket. To get 100k Qantas miles, you need 45k starpoints (I am not sure about the exact conversion; but 45k is my interpretation of the rule.) To get 45k starpoints, you need to put 45000 dollars on the Starwood AMEX card.

Alternatively, if you have any other airline credit card that give you 1 mile per dollar charge. It will require 80000 miles in other program. That also means you need to charge 80000 dollars to get the biz class ticket.

Of course, this calculation does not figure in the one time sign up bonus or special double mile promotion etc...