This should result in them actually fetching from mirrors instead of
gb.archive.ubuntu.com or http.us.debian.org.
d-i keyboard-configuration/xkb-keymap string us
# Download packages from mirrors.mit.edu
-d-i mirror/country string enter information manually
+d-i mirror/country string manual
d-i mirror/http/countries string manual
d-i mirror/http/hostname string mirrors.mit.edu
d-i mirror/http/directory string /debian/
# Download packages from mirrors.mit.edu
-d-i mirror/country string enter information manually
+d-i mirror/country string manual
d-i mirror/http/countries string manual
d-i mirror/http/hostname string mirrors.mit.edu
d-i mirror/http/directory string /ubuntu/
# Download packages from mirrors.mit.edu
-d-i mirror/country string enter information manually
+d-i mirror/country string manual
d-i mirror/http/countries string manual
d-i mirror/http/hostname string mirrors.mit.edu
d-i mirror/http/directory string /ubuntu/