Yes, but don't get on more than 3 hours before the game or Southern will want you to pay. They don't like people using their match tickets to go into Brighton before the game, naughty, very naughty.
Southern Rail are crap. A few games ago, for the first time ever they challenged us about travelling more than 3 hours before the game. I didn't pay for an extra ticket, my son did.
The conductor person argued that people shouldn't be using the train to go into Brighton to do something else...