Depends on your qulity of life expectations. It's enough for food and transport. For going out and enjoying some of the finer things in life you'll end up rather a niggard.
Simples, time for HAROLD'S UNIVERSITY PLAN! Okay buy like a box of cereal, which should last a weekish and they cost like a pound so you've got 99 pounds left. I don't know to be honest, buy excessive amounts of bread and a toaster to carry around with you for lunches? Oh, don't forget the spreads, my dude! And for your dinners, well you can figure that out, be diverse you know don't follow a plan completely. Good luck ,do not perish by rationing too much my dude.
LOL. This is why it's a really bad idea for some people to go to uni right after school. Absolutely no clue about the real world but about to get into £thousands debt and choose a lifelong career. Sounds legit. 100 quid to live off p/w as anything is well enough (after rent/bills) never mind as a student. Wow, just wow.
With catered halls, all you really need is like £20 a week for going out and then occasional extra for clothes and such. £100 is way more than enough. People comfortably manage with just £50 a week and they use that to buy food too.