Cowfold Seagull
Fan of the 17 bus
Annoyingly the 5B bus does go all the way up to the Uni at Falmer but only during 'peak times' - which appears not to include any sporting event of 25,000 people.
True, the last 5B that is extended to the Uni, leaves Preston Circus at 17.14, so you would need to make an early start. I have used it once or twice, and it can take a long time to reach it's destination in the rush hour, over an hour on one occasion I seem to remember.
Much better to take the train option from London Road, or hop on any bus heading south, alight at St.Peter's Church, and change to a 25, 25X, 28, or 29, to complete your journey.