Should I appeal my UCL decision?

okay ik this is probably a ridiculous question but i’m getting desperate 😭

last year, I achieved 3 a levels and this year I’m studying a level biology privately, therefore I have a predicted grade from a private tutor, which wasn’t put directly on my UCAS application. This wasn’t a problem with most unis as I just emailed admissions and they attached it to my application, however, when I tried to send it to UCL, they sent me an email back saying to ensure fairness they don’t accept any documents sent by email and that they’ll request any additional information

so I’m sat here assuming they’ll request my predicted grade over portico because that’s what I’ve been advised, but they literally never requested it and straight up rejected me lol. The predicted grade was A* in Biology which honestly I feel is pretty relevant as I was applying to BSc human sciences and as they say, they view applications holistically and aim to be as fair as possible to not disadvantage other applicants, but idk I’m feeling like I was the only one who was disadvantaged in the process

You were the only one disadvantaged in the process because you didn't put the Biology on your UCAS application.
Why didn't you? You say that "The predicted grade was A* in Biology which honestly I feel is pretty relevant" yet you figured it wasn't relevant enough to include when your first applied? Is that because you made the decision to take Biology A level late - after you'd completed you UCAS application? In that case, you could have simply contacted UCAS via their Making changes to your application after you apply form.
As UCAS say here with regards to your application (which they even put in bold): "You must enter all your qualifications from secondary education onwards – whether you have the result (even any that were ungraded) or you’re still awaiting exams and results."
It's not really fair to judge UCL harshly because they haven't implemented a mechanism which would allow you to circumvent the UCAS process. Given that they say, "to ensure fairness they don’t accept any documents sent by email" it's reasonable to assume that they've had people attempt to cheat before - which is why they want everything to come through UCAS.
Now, I agree with you that the fact that they said they’ll "request any additional information" and then didn't doesn't present them in a very good light. So it would be worth contacting them and pointing this fact out, and asking how they'd like you to provide details of your Biology exam. However, it might be too late for that to have any effect.