Rossco mate,
Wear whatever you feel most comfortable in.
I was at BA Medical in the morning so wore a shirt and trousers but most at the CAA wore casual stuff like jeans and a top. It doesn't matter really coz they soon stick you in a lovely white terry toweling number!
There is nothing to worry about, just have a few drinks of water to flush out the crap in yer bladder in the morning. (stay off the booze for a couple of days prior) RELAX.
You will get Cardiogram, Audiogram, Brain Scan, Medical, Specimen, Lung Function, Eye Exam, Tympanogram (lugs), Chest X-Ray, Finger-prick blood samples,weight&height and thats about it.
Nothing to worry about.
If you are halfway fit and healthy then its a formality. As I said my BP was ballistic! but everything else was ok so just chill!
Let me know how you get on.
All the best
VORDME.