Aside from rent then about £50 a week, this will cover a couple of shopping trips to get food etc, a weekly bus pass if you need one and you can go out one night, pay for entry and have a couple of drinks. Where students really spend money is on taxis, takeaways and nights out, some will spend £30-£40 on a night out and go out 2/3 times a week.
Hmm I've been wondering this. I'm aiming to have something between 80-100 a week to live on which I'm hoping will be in excess of what I could need or want!