Payment method is not available when changing country

What you need know before you change your country or region

When you change your iTunes Store country:

• Update your billing address to one in your new country. When you change your billing address, the primary address for your Apple ID and your other Apple services, such as the App Store and iCloud, automatically updates.
• You might need to update your payment method. For example, only German credit cards can be used to buy content from the German iTunes Store and App Store.
• You won't see the items that you purchased from the previous country’s store in your Purchased section.
• You won’t see the items from your iCloud Music Library that you matched, uploaded, or added from the previous country’s store.

When you make a purchase in the iTunes Store, App Store, iBooks Store, or Mac App Store, the country in your iTunes Store account information must match the country listed on your Apple ID account page.

Find out what to do if you can't change your country or region.

Information above from here > Change your iTunes Store country or region - Apple Support

You are following the instructions on this page when trying to change your country : Change your iTunes Store country or region - Apple Support ?

And by 'UK bank card' you mean that the card was issued by a UK bank, and is registered to the same name and address (including format and layout) as you are putting on your account ?

And you were using those steps, you changed the country/region to the UK and agreed to the UK store terms, and then tried entering your UK billing address and card details ? If doing it on your iPhone, you've also tried doing it on your computer's iTunes ?