make sure your theory is good. if you havent done grade 6, i'd get the workbook and check to learn stuff like auxiliary notes, suspensions, circle of fifths chord progressions etc. if you're doing edexcel, you'll do bach chorales, so you need good thoery skills. composition needs lots of work aswell.
but its soooooooooo good, you'll love doing it. essays in music take time to get used to how to score top marks, but if you're prepared to put the work in, you'll be just fine. a bit like any other A level really.
i hope you enjoy doing music.