So, choosing the best mirror in the /etc/apt/sources.list can be simply done by removing the country prefix before "archive.ubuntu.com" (i.e. "us.").
For a new installation, it should be possible to specify this instruction on the preseed file:
d-i mirror/http/mirror select archive.ubuntu.com
But for some reason, the installer ignores this line and keeps the prefix as the ISO-3166-2 country code, from the locale settings. This is not so optimum for standard installations with "us" locales connecting from somewhere else than US.
Is this a bug, or am I doing something wrong? (I'm using Ubuntu server 16.04)