Hi @Phil Marland ,
Thanks for the question. You will not be able to see the host Azure VM capacity in a region. That being said I have a few options for you.
Option 1: You could write your deployment script to attempt to deploy your build server in UK South in the morning, if it fails due to capacity constraints your script would attempt to build it in a backup region. This is what I would do if I were you.
Option 2: You could open a ticket with support to inquire about getting a form of reserved capacity.
Hope this helps!