alberto,
I do not know why you would choose not to eat - does your form tell you not to? - as I know I certainly didn't change my eating habits when I went for my medical, and as you say you do not want to faint, nor do you want your body to show any signs of stress due to hunger.
RE: urine sample. I am pretty sure you are not allowed to bring a sample with you - who's to say the sample you bring is genuinely yours? - so you are directed into the men's toilet for the urine sample, the end cubicle having a little metal slide panel on the wall, which you open to pass on your sample.
Being new to the kind of toilet with sliding panels, curiosity got the better of me and, whilst in the process of 'filling up', I opened this slide with my free hand. I don't know who got the bigger shock: me or the nurse who was on the other side
. No words were exchanged and as I closed the slide back, I thought "I've learnt from this". When I finished, I opened up the slide, passed over my sample and was rewarded with a smile. I'm still unsure whether this was for the sample or the sight.
VC10 Rib22
