most, if not all, of the major distro repos are mirrored. first port of call is usually mirror.ac.za. the IS ftp server is usually a good place to go as well.
Yes, I also see the default server is being routed through local again - a while back I had to change my server so that the updates wouldn't use up my international bandwidth. Oh yes, Windows updates are also local (not that I use Windows anymore).
Usually yes, but I had a bit of trouble over the week-end to get Kubuntu to update, so under South Africa you can further specify any local server you choose to use, they list a few but you can always add your own as well.
Just be careful of the DNS servers that you use. za.archive.ubuntu.com has nodes both in South Africa and Europe. If you use an international DNS server like OpenDNS or GoogleDNS then you are automatically redirected to the European server cluster. If you use the local DNS of your ISP then you will have no problems and all your updates will come from one of the local clusters.
There will be no issues if you use mirror.ac.za none at all. Only problem though is that the mirror is sometimes not always as up to date as the international mirrors.
I have always been happy with mirror.ac.za's mirror. One thing I have noticed about Ubuntu though on my server installations - if you specify South Africa as your location, it will load the correct mirrors, but not for security updates. I just go manually edit my etc/apt/sources.list to the local ones, and all comes through on the sweet free nectar that is local BW.
security updates for some reason bomb out when Repo set to "south african severs" ; if you set it to mirror.ac.za or leg.uct.ac.za, the the updates fetch security updates locally. That has been my experience.
S