Without customising the Ubuntu live image, is it possible to select a mirror during installation? If not, what are the options available? I can think of customising the image, or somehow using preseeding with Ubiquity.

The annoying thing is that I am booting live Ubuntu over the network (via PXE booting), so the network connectivity is assumed. This causes Ubiquity to try to fetch some things from the mirror, even though, by its admission, it doesn't have "internet connectivity", since we are behind a campus proxy. Because of this, it spends a lot of time trying to fetch something and we have to manually skip that part. We do have a (official) mirror within campus, so we would like to use that.

I'd like a way that doesn't involve customising the live environment, so that I can suggest that as a way for students who use their own USB drives.

(Here is, for example, a question about another problem from live booting over the network that I am facing: Ubuntu Live over PXE Boot sets manual config for networking)

Here are a couple of screenshots from a test on a VM:

Here are a couple of screenshots from a test on a laptop:

Note the usage of in.archive.ubuntu.com, even though, following @noleti's advice, I had set ftp.iitb.ac.in as the mirror:

This is currently on the wishlist for ubuntu, since 2008, it has been reported on lauchpad, it is bug #202052.

It is also unassigned, so I wouldn't expect it any time soon.

Sorry for it not being better news, but without customising your Live image, it isn't possible.

Updated: includes more info from comments discussion:

It depends on whether ubiquity is using /etc/apt/sources.list for the install

Possible solution: manipulate /etc/apt/sources.list

During installation, you should be able to edit the /etc/apt/sources.list (as root) to redirect to your mirror.

deb ftp://ftp.iitb.ac.in/distributions/ubuntu/archives  trusty main restricted
deb-src ftp://ftp.iitb.ac.in/distributions/ubuntu/archives  trusty main restricted

More info here. Then, run an sudo apt-get update.

By redirecting the call to the mirror

It might be possible to redirect the hard-coded communication with the mirror. Options for that: /etc/hosts dns manipulation to direct to another server with same protocol and directory structure, or by using a local transparent proxy. In your case, assuming you can set up the http mirror on the same machine as your ftp mirror: add the following line to /etc/hosts

103.21.126.20 in.archive.ubuntu.com