I have £30 account credit on my account on the app store - how do i withdraw this to my bank?

Apple ID account balance has to be spend on the App Store (apps, or in-app purchases) or iTunes Store (music, movies, tv shows, audiobooks, ebooks) or subscription services. It could be spend in the online Apple Store on hardware in select countries. It can’t be transferred outside the Apple ecosystem; it is store credit.

What you can buy with your Apple Account balance - Apple Support

Check your Apple Account balance - Apple Support

We probably can help, but you'll need to post a more full question about what it is you're trying to do and where you're running into difficulty.

Regards.

Posting a screen shot of what you see and where you are seeing it may also help.