Just because you bought something X weeks ago does not mean it was posted the same day.  To start with the payment can take up to a week to clear if you used e-cheque as a payment method.  I have had on several occasions "instant" payment take several days to clear.  And not all sellers rush out to the PO the moment the payment clears.

In UK, in preparation for the Olympics, there is heightened security everywhere, including with mail going on planes; so there is a great possibility there is a delay with mail there.

And if parcel was coming from country NSW to SA by road, that would definately take a week.

I am sure that your local PO is not deliberately sitting on your mail.  They deliver it when they get it.

By the way, I am sure there is no such a thing as GPO anymore 🙂