Prior to emailing the list, I tried several
combinations of hosts
and paths before giving up and just using
deb.debian.org. Even
after your explanation above, I'm unsure how to derive the correct
cached source URL, given that the official backport instructions
don't give a source URL which uses
ftp.uk.debian.org or
backports.debian.org (or any of the other mirrors listed in the wiki
page). Maybe there's an underlying assumption that a mirror can be
used simply by swapping out the original host name for the mirror,
but even if that's the case I think it should be documented
explicitly because it's not uncommon for mirrors to be hosted at a
different URL path to where they were on the original host.
It's another one of those things that's only documented in a small
number of obscure places, but you can find out what dists are
available from a mirror host by appending debian/dist to the URL and
opening it in a web browser.
For example: