Can an employer refuse to change my working hours to fit round my education?

I am going to university in September but living at home and commuting in. I cannot afford to leave my job as I will need money to commute and have some bills to pay as well. I am only contracted for 16 hours a week but since I be at uni most of the week and don't know when my lectures are, I want to change my hours to weekends or do one day at the weekend and 8 hours over 2 separate nights. I work for the a leading (ish) chain of convince stores in the UK. Can my employer refuse my request?