Navigate to the checkout editor (Online Store > Themes > Customize > Checkout)
Click on the UK Postcode Lookup block on the center preview pane
Click the trash icon as shown below
Save changes top right of the editor.
NOTE : Removing an app block only removes that element of the extension from your store. It will prevent further lookups and charges associated with that app block. If you have other app blocks installed, such as Email Validation or Address Auto-Complete, you will continue to see usage and charges for these. If you have a recurring subscription in place for the app you will need to cancel that separately through your store settings.
If you want to fully uninstall an app and all it's app blocks or extensions you can follow the steps below
Navigate to your store Settings > Apps and Sales Channels
Click the to the right of the app you want to uninstall and click the Uninstall option
Uninstalling the app will remove all the functionality from your checkout immediately if you haven’t already removed the app blocks. If this change is made in your live environment, the app will cease to appear in the checkout or perform any further validations. No further single transaction app charges will be generated. Check your subscription settings however, to ensure any associated monthly or annual subscription is also cancelled.