You don't necessarily need a prewritten essay to do well in English. I suppose if you've next to no essay writing skills then maybe it'll be the path you venture, but seriously, with English you NEED TO KNOW YOUR CONTENT! <--- I learnt THAT the hard way
What are you going to do if they didn't specify an essay and instead asked you to write in another text type? Then if you didn't actually know and understand what you were writing about then you wouldn't be able to mould any answer to the question and thus not get the marks you're after. It'd be a lost cause.
Pre-Written Essays are usually essays that you've worked on the entire year. I had one for Imaginative Journeys yesterday that was easily mouldable (and I understood the concepts and arguements I had) so that I could edit and fix it up on the spot to answer the question instead of just regurgitate some preprepared response that doesn't answer the question. It's a risky business. Anyway, back to my story, lol.... I wrote this essay at the beginning of the year, gave it to the teacher, wrote it at Half Yearlies and Trial etc and just kept modifying it and making it better using the teachers suggestions until I ended up with a pretty decent essay. ----> This also helped me understand what they look for in the essay and what the course requires you to have a sound grasp on.
You can't "make" a prewritten essay two days before the exam and expect to ace it. My suggestions would be to do past paper questions and make sure you understand all the points for each module that you can discuss, formulate a paragraph or some phrases and sentences you want to use etc.
Good Luck