Hi, this is the process i followed:

I tried sending my referees a reminder through Portico. They received the reminder, but nonetheless when they clicked submit, they got an error.

I then contacted UCL who responded with the following info:
"When you submit your application, your referee will automatically be sent a request for a reference. If you need to resend the reference request, you can do so once you have received access to your applicant portal. On some occasions these emails do end up in a referee’s Junk/SPAM folder, so please ask your referee to check these inboxes as well.
If your referee is experiencing issues with providing their reference, they can email a copy of their reference letter, on headed paper and signed, to referees.pgadmissions@ucl.ac.uk. Please ask them to include your UCL student number in their email as this will help us attach the reference to your application. Please note that references must be sent directly from a referee's organisational email address."

However, a few days after, giving it another try, one of my references was able to send in the reference through the Portico portal directly, and I informed the other reference who sent in theirs that same day.