In the past sussex has been relatively lenient in my experience about letting people in if they miss theyre main offers. HOWEVER; they are doing this unconditional offer system this year for the best students that they really want. Do bear in mind, you cant expect them to give it to you if you miss your firm.
Sussex has a wonderful social sciences department. But please dont hedge your bets on sussex letting you in, I know 6-7 people who got rejected for missing theyre offer, 2 of which were rejected from the politics department. (One doing politics and IR, the other politics and economics)