Remove the credit card from your account first. Then you can spend the £10.
- Launch iTunes, go to iTunes store.
- Click the "down arrow" next to your email at the top right side of the page and click "Account."
- Enter your username and password and click "View Account" to log into your account information.
- Next to your Payment Type, click "Edit."
- Select the "None" button and click "Done."
You can confirm your card has been removed by going back to the Apple account info screen and under payment type, it should say that there is no payment information on record.
This will make your account work as if it was started with an iTunes gift card.
From there you should be able to switch over the the aussie store.