Re: The connection to host imap.virginmedia.com on port 993 failed.
Same problem here on two iMacs (one running MacOS 10.15.6 Catalina and the other running MacOS 10.14.6 Mojave).
It does connect eventually, but is very slow and intermittently drops the connection, giving the message "The connection to host imap.virginmedia.com on port 993 failed." and the Status is marked as "Offline". I have the same issue whether the server connection settings are set to "Automatic" or manually input.
Have checked VM's Mac email instructions (the screen shots are out of date btw) and my preferences are correctly set up: