For any interview, its good practice to dress smart for it, even for something so laidback such as F&B or retail.
It shows you care about the job, and want to give a good strong impression.
Congrats on the interview.
I haven't had one for a student's union, but I used to work in F&B for a visitor attraction, and Produce at Asda, so customer service skills are critical here.
You'll likely be engaging with students, providing them food and drinks in the union bar, with the likelihood of cleanup / wipedown sections (cleaning tables, counter workstations) etc.
Just be yourself, state that you have a passion to learn to create alcoholic beverages and deliver the best customer service you can to the students at the uni.
If they ask you any confusing questions, you have the right to ask them to rephrase it, or explain it differently to a way that you understand it best.
Good luck!