From what I've observed, I suspect if you rocked up at 5pm you could park (for free) in Stanmer Park itself - there seems to be a lot of that going on nowadays.
At the Uni, we've only ever been right at the very top/back, closest to Falmer itself. I don't know how early you need to be there to...
c100 Uni
c150 Bridge
https://tickets.brightonandhovealbion.com/en-GB/events/parking%20v%20man%20city/2023-5-24_20.00/american%20express%20community%20stadium?area=0706bd0e-6a64-4c3a-bde7-20a23e1924a5&type=ba