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If you are not able to change the region for whatever reason, try changing country under Apple ID.

On your iPhone, iPad, or iPod touch

1. Open the Settings app.
2. Tap your name, then tap iTunes & App Store.
3. Tap your Apple ID, then tap View Apple ID. ... 
4. Tap Country/Region.
5. Tap "Change Country or Region." If you don't see "Change Country or Region," contact Apple Support.

Your physical location, payment method billing address, and the store you're using must all be in the same country.

Or try this.

Settings > General > Language & Region > Region > [Select United Kingdom]

Based on your country of residence or country you have selected under your Apple ID --> Payment methods that you can use with your Apple ID - Apple ...

Ask your bank only one thing, i.e. if the Debit/Credit card which is issued to you is valid internationally.

Rest everything check at your end.

Check the following

1. The payment method that you are adding is of the same country as that of App Store Country on your iPhone
2. You have valid payment plan / debit / credit card from an authorised bank from the same country as that of your Apple ID
3. Learn more about family purchases and payments.

What country are you in?