Who's money are you spending?
Seriously the CAA or IAA are not going to worry one iota what you are wearing, for the record when I took my inital in Sept 2003, I wore jeans, trainers and a tee shirt.
For the record, when I have my renewal, I still wear Jeans, shoes and a shirt. Thats me comfortable. You should be comfy, not stiff. Remember this is not a interview.