To be honest, £17 a week is enough to live on, but it is not an easy life. You just need to cut back a lot. A LOT. And budget like crazy. If that means not going out at night, buying basic ranges at the supermarket or buying in bulk so that you have more food for following weeks, then so be it. The stereotype of poor students is a stereotype for a reason.
When my partner was unemployed for a while a year or so ago, my student loan had to cover both of us, which meant that after rent and bills we had about £15 a week for two people at a stretch.
The student finance system here is fair, and because of that asking for more money would yield you nothing. If it were possible, then every student would be asking for more. Being a student teaches you more than your degree - it sets you up with valuable real life skills like budgeting and surviving on the bare minimum.
There are overdrafts there for a reason too. You can also look into getting a part time job. Unless you have a genuine case then asking SFE for more money will not get you anywhere. Your parents not helping you is, unfortunately, not a genuine case.