On my network i use apt-cacher-ng on server and add
Acquire::http { Proxy "http://[10.48.21.23]:3142"; };
to /etc/apt/apt.conf on clients.
So apt-get can't download file through proxy with wget.
apt-cacher-ng shouldn't interfere with the actual package download. The problem with these two packages is they don't actually contain their payload. They are both lightweight packages that contain scripts to download extra data with wget when you install them.
This is simply for legal reasons. The extra files licenses' conflict with the repo's.
If you need to force these downloads through a proxy too, you can, either by setting up wget's proxy settings in /etc/wgetrc:
http_proxy = http://proxy-ip:8080/
Or you can use the longhand command to pass the proxy through to it:
sudo http_proxy=http://proxy-ip:8080 apt-get install flashplugin-installer ttf-mscorefonts-installer
See: https://bugs.launchpad.net/ubuntu/+source/flashplugin-nonfree/+bug/362970
The proxy address I'm talking about in these solutions should not be apt-cache-ng - that's only for caching package downloads. I'm talking about the same proxy that you'd use in your browser, for example.
