I doubt you'd need it, lots of the students come from England. But about 40% of Aberystwyth (town)'s residents speak Welsh fluently, so it you're up for learning it, it may be advantageous. One of the reasons I'm planning on going to Aber is to improve my Welsh, I live in a very anglicised area of Wales, where only a minority (although a strong minority) speak Welsh, and I haven't spoken to someone in Welsh since May 2010 so it's really gone bad!