I think you'll find that the most terrifying part is where you part with your money!
As far as I can remember, you'll do ECG, EEG, lung function (you blow into a tube), fairly comprehensive eye test, audiogram, blood test (I think they're doing anaemia and lipid profile?), blood pressure etc (I probably forgot quite a few others though). I don't know how long it takes nowadays, as it usually involves certain amount of hanging around. None of the tests is scary, don't worry too much, and relax.
They should be able to tell you if you have met the standards on the day if you ask, although they will probably say that it's subject to having ECG looked at etc etc.
Good luck.