Difference between POP and IMAP: POP will download the emails to your phone and remove them from the server, leaving you royally f***ed if you want to use any other email client, including webmail later on
IMAP will simply read from and write changes (deletes, mark as read, etc) to the server. Would recommend IMAP over POP anyday, especially for a phone.
As for signing in with K9, it's kinda hard to pinpoint the problem. There's a lot of variables and they all have to match the server's capabilities to the letter.
Top tips, apart from the obvious ones like having the right password and email server URL:
- Try username with and without the "@caledonian.ac.uk" bit
- In security type, try both SSL (if available) and TLS (if available). 98% of email servers out there should require one of those.