Landscape-only Solution
There is a feature in Landscape called 'Repository Management'. This feature is a bit complex to use and is more intended for people who want to stage updates across all their lab infrastructure. But, feel free to look it over at the following links:
Just keep in mind, it lacks a user-friendly view in Landscape. That major caveat aside, it does work and is seamless from the client point of view -- even supporting private repositories (https).
Squid Deb Proxy
If you are just wanting to do some proxying of the ubuntu archives though, you should look into squid-deb-proxy:
# On the landscape server or any machine your client can see with internet access:
sudo apt-get install squid-deb-proxy
# On the landscape client (you may have to do this by hand the first time, i.e.,
# download debs and use dpkg -i):
sudo apt-get install squid-deb-proxy-client
If you are using public repositories (like the Ubuntu archive and Launchpad PPAs), this should just work. But see the docs and the multitude of questions/answer on squid-deb-proxy here and on the forums if you need help getting things configured.