When I was saving haaard for travelling I was managing to spend £20 a week (excluding rent), and still go out twice a week, and get bladdered one of those times. I wouldn't recommend that, but just know that you can live on much less then your budgets if you ever need to.
My normal budget:
£20 a week food
£40 a week going out
£10 a week records (that's what I wanna spend any excess money on, but put that tenner down for eating out, clothes, cds, etc)
£10 a week course expenses