Amazon Calculation

Exchange rate
1 Australian dollar = 0.625184899 British pounds
1 British pound = 1.5995268 Australian dollars
VAT = 20%

Cost of OGS bluray = $8
Hence Cost including VAT = $10
$10 in British Pounds = $10/1.5995268 = ~£6.25

So Amazon bluray needs to be below £6.25 to be cheaper, at parity we can assume free shipping is better than £25 free shipping restriction