As far as the School of Computer Science goes, just your university ID card. If you're a UK student and register online by noon on 6th September, it will be posted to you within 3 weeks. Otherwise, you can collect your card during Welcome Week from the Welcome Hub. Please do this on Monday, as you'll need your card when you come to us on Tuesday for the first induction event.
In addition, the university publishes a useful packing list, which will be particularly valuable to international students. If you're British (or have snuck into the country), it's definitely a good idea to have your passport with you. You'll need it to register with Worklink, which is the system through which all university employment is paid.