Comment 14 for bug 1135163

Thank you Philipp, with mirror/https settings everything is now fully going over https.

Here's our working configuration with unauthenticated https installs for 14.04 in case anyone else comes across this:

PXE Config:
KERNEL pxelinux.cfg/kernel
APPEND append initrd=pxelinux.cfg/initrd.img vga=normal fb=false auto=true priority=critical auto-install/enable=true debian-installer/allow_unauthenticated_ssl=true url=https://buildhost/trusty/preseed.cfg.pl media=https://buildhost/ubuntu/

Preseed Mirror settings:
d-i mirror/protocol string https
d-i mirror/country string manual
d-i mirror/https/hostname string buildhost
d-i mirror/https/directory string /ubuntu/
d-i mirror/https/proxy string
d-i mirror/https/mirror select mirror