Here is a working snippet of a preseed.cfg which works with apt-cacher-ng out of the box:

# 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 GB
d-i mirror/country string manual
d-i mirror/http/hostname string
d-i mirror/http/directory string /debian
d-i mirror/http/proxy string


You will need to change the IP of the apt-cacher-ng server to the correct IP/hostname - in the above example.