Eddy, it's really not much to be worried about at all (Besides the cost of it and having to shell out more £s each year to revalidate it!)
You visit various CAA medical types in the few hours that you are there. Nothing to get worried or stressed about. Usual things you would expect - eye test, hearing test, lung capacity, general medical check up. It's actually quite funny to be honest. The CAA get you to dress in a white robe and prance around the Medical department going from doctor to doctor and checkup to checkup. As I say, all over in a few hours.