(had problems with proxy setting before, tho i can't remember what)
### Mirror settings
# If you select ftp, the mirror/country string does not need to be set.
#d-i mirror/protocol string ftp
d-i mirror/country string manual
#d-i mirror/http/proxy
ofc this leaves the proxy in /etc/apt/sources.list after install
but i didn't have a problem with that
you could just sed it out or something in the late_command