Partially redeem Apple Gift Card on App Store After using £200 of the value of my Apple Gift Card in the apple retail outlet(physical shop), I now have a balance of £500 of which I'd like to put £100 of that towards my App Store credits so I can buy iCloud storage, apps, etc. Here are my questions:

In the dialog to redeem a gift card on the app store I'm not sure if I can partially redeem just £100. Is this possible? I'm wary of going forward with this in case all £500 is redeemed to the app store, which would be needless.

Is it possible to buy iCloud storage using balance from a Gift Card?
You cannot partially redeem using the redeem page you're referring to. I don't know whether there are other methods that allow this (such as calling) but the redeem page will redeem the entire card.

If you have a gift card balance, all subscriptions including iCloud Storage will bill to the gift card balance on your account.