Trying to update payment method from Guernsey post code

Regarding the subscriptions, you need to cancel them, and then wait for them to end before changing region.

Make sure all these steps have been followed-

• Spend any balance remaining on your Apple ID. If you're not sure whether you have a balance, check your Apple Account balance.
• Cancel your subscriptions and wait until the end of the subscription period. 
• Wait for any memberships, pre-orders, movie rentals, or Season Passes to complete, and wait for any pending store credit refunds to process. You can cancel a pre-order.
• Make sure that you have a payment method for your new country or region. See what payment methods you can use.
• Redownload your apps, music, movies, TV shows, and books on any devices that you might use to play or read them in the future. Some types of content might not be available in your new country or region. Learn what types of content and services are available in your new country or region.

Change your Apple ID country or region - Apple Support

You need a valid credit card issued by a bank in the country the store is in and you need a billing address in that country.