This is a very difficult question, particularly as with medicine, you are limited to 4 instead of the normal 5 UCAS applications. There are over 30 medical schools that all have very different requirements. For example, some focus heavily on the UKCAT while others are more reliant on GCSEs, AS levels or predicted grades. The first this to do is to look at your own personal circumstances. Are you happy moving to a university far from home, or would you prefer being close by? Then its also very important to look at the type of course offered by the university. Some universities are very PBL heavy, while others have a predominantly lecture-based way of learning. Lots of people neglect this point and focus so much of getting into medicine that when they actually do, they find that the method of learning does not suit them at all. So it is worth bearing this in mind.
This should help whittle down the list of universities to a more manageable number. Then its a good idea to look at the entrance requirements of each university's admissions page. Finally, you can always email the admissions tutors with your personal circumstances and I have found that they are only too happy to help.