The medical consists of a questionnaire about you health, medication and alcohol consumption.
On the day, there is a blood test that will show your CDT level that is used by DVLA to assess if you are alcohol dependent. It does not test for alcohol as such, you could have a pint of beer on your way for the medical and still pass, but they look at your score as indicative of your dependence.
Above 3.0% is a strong indicator and will see your application turned down. 2.3-2.9% will prompt enquiries with your GP and below that indicates that you are not alcohol dependant, but they may still decide to make enquiries with your GP after assessing your questionnaire.
there is not a requirement for a urine sample, but one will be requested if there is evidence of drug misuse, and we do see reports of people being asked for urine samples routinely with no obvious reason why. Similarly there is no requirement for an eyesight test, but some people report this as being a part of their medical. There are other general bits like recording height and weight and the doctor will "palpate" your abdomen to feel for a swollen liver.
The results are sent back to DVLA and their medical advisory panel made a decision on you getting your licence back. The timescales for this seems to vary quite a bit, there was a recent result reported where the licence was sent back in 8 days, others have still been waiting weeks after the actual appointment.
The cost is in the region of £96, payable as cash or visa on the day, or usually a week or so before if you pay by cheque.