Generally speaking, if someone wants to study medicine in the UK, they can get in through 2 paths.
Route 1. A-levels (school leavers): These students usually apply for the 5 year MB BS course, (or 6 years MBBS - including a foundation year that is provided by some universities like King's College for students with grades less than the entry requirements for the 5 year MBBS). Usually grades are not the only determining factor for one to be accepted into med school. Enough work experience at a hospital setting or a GP is also required by most if not all universities (occasionally, working in a caring environment is accepted by some).
Personal statements with the UCAS application form are also very important. They need to show evidence of commitment to medicine and that one understands and has analyzed a career in medicine. UCAS forms must also contain a reference from a personal tutor stating predicted grades. Finally, from 2006 onwards any one wanting to apply for medicine has to sit an aptitude test in a test centre (not at the University) and the results from this test are used to call students for interview at many universities. These aptitude tests are UKCAT, GAMSAT-UK, MSAT, BMAT... and i am not sure if there are many more! - but i think these are it. Students that fulfill all the criteria above and score good at the aptitude test are then called for a structured interview. If they score well, they are admitted (but sometimes if they are unfortunate, they are put in a waiting list for deferred entry).
Route 2: for Graduates of all disciplines: people who done any sort of degree Arts, BioSciences, Engineering, etc.. and have decided medicine is their career after all... well, they can apply to study the 4 year MBBS through the Graduate entry programme (GEP) and become doctors in just 4 years.
Usually the entry requirements vary from university to another. They need to be predicted a good honors degree by their tutors i.e. 1st/ 2.1/ or sometimes even a 2.2 degree is accepted for instance at St.Georges University of London & Nottingham university. They also need to complete their first degree of course before getting into med school. Just like route 1, GEP students also need to sit the aptitude tests and apply in the normal way through UCAS and produce top personal statements.
