I would suggest that you buy your tickets when you get there.
a) You will want to be going to the same events as your friends and perhaps your new flatmates are going to an event on another campus that night. Best to wait.
b) If your dept hasn't started teaching, you will have very little to do during the day on Freshers' Week as there are only so many induction lectures you can go to. If you wait til you get there it will give you something to do, and an excuse to explore the campuses.
You will probably be given a tour when you get there but people are very helpful (especially as the Strand campus is like a rabbit warren!). So don't worry about not knowing where you are going for the first few days! Tutors, in general, are very understanding.
Best of luck for everyone starting.
If you have any questions, feel free to pm me.