An example: 20th Anniversary Edition of Nirvana's Bleach @ JB HiFi never less than AUS$35 (in store last I saw it it was $41).
20th Anniversary Edition of Nirvana's Bleach @ Amazon.co.uk = £4.49, and when you actually go to purchase it, it is actually £3.74.
Even with postage total order is AUS$11.84.
This has very very little to do with GST or Custom's Duty or any other kind of cost.
It has to do with the ability to find the cheapest products, anywhere in the world.