A student overdraft is a lot easier to get than you're making it out to be. And numerical credit scores are meaningless. You wasted £6.99 or however much that cost you to get.
But yes, if you don't need it, put it away in the highest interest savings account you get. Just make sure the student account will let you leave it at -£500 or however much you get for any length of time. You might have to actually use it in some way to show them it's being genuinely a student account (e.g. paying your loan into it/having X amount of transactions in a time period)
put it in a ISA, you will make even more then
The largest overdraft your likely to get is about £1,000. 3% interest on an easy access account so you'll make £30 a year.
How you got £100-200 I've no idea.