Why don't you just get the 25 bus from Portslade station before the match? OK, it's not the fastest route to Falmer, but then again you wouldn't have to play Train Roulette and rely on trusting Southern to run a half-decent service.
Whatever the rights and wrongs of the thing, it IS a bit naive to just rock up to the station half an hour before kick off and expect to get straight on a train, when there is a near-capacity crowd expected. A good rule of thumb is to never under-estimate Southern's ability to f*ck up your day...