Best bet would be to cancel completely.
Then just ring them up (0800 039 1010), and say you're looking to buy a Black MacBook for your University course.
They'll ask for your University and course, just tell them... but try and engage the customer service rep in conversation.. a witty joke is always good.
They'll probably not ask for proof, if they do you can a) bail out b) offer to send an acceptance letter.
Not sure ringing them up now that the order has gone through is exactly a good idea.. they may ask for proof, and it could complicate the order.. best to start again me thinks