Similarly to the concept of official mirrors, which are trusted mirrors known to work reliably, we should also have the concept of "official country mirrors". These are the mirrors that other mirrors on a given country are instructed to mirror from, and they usually get an country-iso-code.{archive,release}.ubuntu.com.
Like we do for official mirrors, there should be a way for the mirror owner to indicate that they want his mirror to be an official country mirror and if the distro owner agrees (by approving that as an official country mirror) it is then assumed to be so. It's probably a good idea to have a separate page where we list all official country mirrors and highlight them somehow on the main mirrors listing page.
We still need to decide if we're going to implement a workflow for users to make their mirrors official candidates for a given country and then let the admins approve/reject that or if we're going to support only a way for admins to flag them as official for the country.