Basically, an overdraft allows you to take money out of your account that you don't have. So if you have £50 in the bank, and you need a book that is 56.99, you can still buy it, and it will go into your overdraft, so you'll have -£6.99 in your bank.
When you have a normal current account they usually charge you for having an overdraft, so they let you use it but they put interest on what you owe, so instead of having to -6.99 you would have -7.50 or something like that. 0% interest means you won't get that.
Santander are offering some money right now if you sign up and pay in a certain amount each month. Their overdraft is also pretty good, but will you need it? And I believe you have to pay a certain amount of money into your account before the overdraft is activated.
Co-op I don't know much about, but again they seem to have an okay overdraft. The only thing I would say when choosing an account is, what kind of banker are you? Are you always in credit? Would you rather have an overdraft for security? Are there going to be times when you need to pay your accommodation but you don't get the student loan til a few days later?
And finally, how are their telephone/online banking services? Apps? And it's rare that you need to go into a branch, but just in case, where is the nearest branch to you?
Quote me if you need any help, I like bank stuff