There's a few inconsistencies in what you describe so I'll let you see if this covers your questions.
This article describes how to change your country setting and the requirements and results of doing this:
Read how to "Change your Apple ID country or region" click here --> https://support.apple.com/HT201389
Here are specific issues and how to deal with them:
● Subscriptions
Make sure you have canceled all your subscriptions ( https://support.apple.com/HT202039 ). However, subscriptions will then still continue until the expiration date. You need to contact Apple and request any ongoing subscriptions you have be ended immediately if you do not want to wait until they expire naturally.
● Spend all your Apple ID balance
One way to zero your balance is to add a credit or debt card as a payment method to your Apple ID and buy items just in excess of your remaining Apple ID balance. This will use up all the balance and any remainder will get charged to the card.
The other way is to contact Apple and ask them to keep any remaining balance.
● Pending purchases and refunds
If you have pending purchases, or refunds in the process of being returned to you, you will have to wait until these clear.
For items where you need to contact Apple to resolve an issue:
Click here --> https://support.apple.com/choose-country-region/media-services Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.
or:
Contact Apple for support and service --> "See a list of Apple phone numbers around the world." Click here --> https://support.apple.com/HT201232
As for using a British card in Austria:
From: Payment methods that you can use with your Apple ID; click here --> https://support.apple.com/HT202631
"Make sure that the billing address for your payment method matches the billing address for your Apple ID. In some cases, you might need to call your bank or financial institution to authorize international or online purchases. You might also need to confirm that the mobile number associated with your payment method is correct."
You apparently can use a card issued in a different country but it has to be billed to you at an address that is in the country to which your iTunes and App Stores are set. Your British card would have to be billed to you at an address in Austria if your account is set to the Austrian store.
