I turned up for my CPL skill test in my usual natural-fibres "uniform" of flying cotton trousers and cotton polo-neck, my instructor looked at me with horror and quietly asked me if I had "proper" clothes to change into. I didn't, the examiner didn't seem to care and I got a first time pass. I have yet to own a white shirt with eppaullettes, and suspect that I may never do so.
Kinda likewise. I showed up to my CPL test in a pair of jeans, Megadeth t-shirt and Converse All-Stars. My instructor (a top guy, incidentally) asked if I had a shirt I could put on... I asked if it would help me fly any better.
If I were going for a job interview I'd be suited and booted, but when I'm paying over £750 for a CPL skills test I'll damn well wear what I like.