Do you do A level music? If you do A level music, you will definitely have to do some basic harmony work for your AS exam in any case...
Trying some grade 6 and 7 ABRSSM theory would also be quite helpful too
I would definitely try some harmony/counterpoint stuff before you go for an interview though.
When I was thinking about doing Music at university my piano teacher said that I would have to be comfortable with grade 6 theory as well as some other harmony/counterpoint work
You should ask some teachers in your school if they can check your work - or how about if you have an instrumental teacher, ask that person if they can go through your work?
By essay/written work, check what the college would like you to send in though. It would be terrible if you had to send something in on short notice. This year, the college wanted the following from Applicants for Music:
" two recent marked pieces of work; a piece of harmony and/or one of your own compositions, AND an essay
This may change from year to year but hopefully that will provide you a good idea of the kind of work you will be expected to submit.
But yes, no written tests ftw!