Leicester A100 / A199 2021 Offer HoldersWatch
In the same boat as you are! Just sent an email asking them about this since we can't really attend the offer holder day without the ID they've provided us in the offer letter/email
The other day, I got an invite to book a place for the offer holder's day and the ID I registered with was just my UCAS ID. Maybe if you have the email about the offer holder day, try registering with your UCAS ID? I'm not sure if there's another unique ID we get on an email after getting an offer.