Well, after no response on here, I finally sorted it.
Like I mentioned in my original post, I was able to log into Virgin Media, just not my email, and it wasn't syncing through Outlook or through FairEmail on my Android phone. So it wasn't my password.
I decided I would go into my profile and change the password. That went through, but still I couldn't access my emails and Outlook had "issues" trying to use the new password.
So I thought fine, I'll change browser. Went to Chrome, which is my backup (I'm a Vivaldi girl, but that's Chromium based too so I don't know what gives) and has never been to VM's site before. Logged into VM and the entire screen is different. Clearly some update has happened on it. I clicked through and tried the dreaded mailbox... and it loaded.
Hopped back to Vivaldi, logged out of it there, and then it went straight back in like nothing was wrong, with the new layout. Outlook and FairEmail then accepted the password too and it was all fine.
Dare I ask if this entire episode was to do with an upgrade to the VM site once you've logged in, and what it ought to have done is asked you to change your password upon login with the old one?
Either way, my issue is finally sorted. If this helps anyone else out having the same problem, then hooray!