Its expensive because everything in aviation is a rip off! I had my initial a couple of months ago. They want to know -
Hearing - across 500-3000hz under 20db damage (allowed 35 @ 3000)
Kidneys - not diabetic
Eyesight - not colour blind and to a reasonable standard, with lenses as required.
General look over - dont worry about this. I pickled my liver through uni and didnt make a damn difference. Dont do it now mind.
ECG - looking for heart murmours etc.
Haemoglobin - check for iron deficincies. Youd know if you had this.
Cholesterol - check purely for your sake. If high issue warning. Stay off the kebabs and pizza!
Peak flow - asthmatics are allowed, but not chronic asthmatics. Need to peak flow somewhere over 350. If you cant make that you can barely walk in the CAA building as it is.
Remember people are passing the initial well into their 40's! renewals are far less strict.
Sorry for the sarcasm, but anyone should pass unless they have a condition that they would obviously be unfit to fly in the first place.
Try
www.caa.co.uk, and click on medicals for further information, or call their Gatwick number on 01293 573700, they are very helpful if you have any questions. You can also book your medical on this number.
Good luck!