Can't think of any better course of action than the one you've described, ie to throw yourself upon the mercy of the accomodations office or perhaps the Students Guild (they do crisis loans etc but I wouldn't expect much given that it's your own fault).
Most practical idea would be to try to convince your bank to increase your overdraft by the amount you need? If so you could just increase your OD by that amount for approx nine months, which at 15% interest would cost you about £30ish quid.
Alternatively find a friend who would be willing to pay the University the deposit with explicit instructions that it is to be paid back to THEM (eg direct to their bank account or onto their debit card), that way they the Uni would owe them and not you.
All accomodation requires a deposit, you'd be paying twice as much upfront with a private landlord. Plan better next time, and read stuff more carefully, but I hope you get through OK!