Based on what the SU says, I'd advise getting the wristband because it will give you priority over people who don't have a wristband when showing up to events. If there's an societies/sports you're interested in book onto their taster sessions as that's something you can go to alone and meet people there (flatmates may have diff interests). Then if you only want to go to evening SU events with flatmates you can see about that once you're talking with them. But if you want to do things you can just get a ticket, you'll meet people there.