I had a similar query last year as I didn’t take film either. In the end I wrote a review of a film I saw at the time in a kind of critical style with a tiny bit of textual analysis. It was very basic stuff, they’re just looking to see if you’ve got the skills. I’m at Warwick now and there’s a real mix of people who did take it for a level and people who have never studied anything like it at all, so don’t worry, what you write will be great I’m sure!