The education system in England and Wales is often spoken of as divided into Primary Education (5-12), Secondary Education (12-18) and Higher Education (18+). Education is compulsory by law for all children up until the age of 16, and there is a national exit exam at this stage, formerly known as 'O' Levels, now known as GCSEs (General Certificate of Secondary Education). The teaching curriculcum up to the age of 16 is laid down by the Department of Education in the National Curriculum, and this therefore closely determines the content and scope of education up to this point. Students who choose not to leave Secondary Education at age 16 after GCSEs, but instead continue to 18, will usually confine their studies to 3 or (occasionally) 4 subjects, in which they will have final exit examinations at the age of 18. These are known as 'A' Levels. The grades scored at A Level, and the nature of the subjects studied, determines and limits the choice of subject that may be studied in Higher Education (= University) and the quality of the institution where such study may be undertaken.
The education system in Scotland is similar as far as I know, except the names of the exams are different.
Medical Schools constitute one form of Higher Educational establishment, and there are about 30 Medical Schools in the UK. Most have an annual intake of around 100 new students, and the course usually lasts five years at the end of which there is a final exit exam ('Finals'). Entry to Medical School is normally made at the age of 18, following on immediately from 'A' Levels. However, all UK Medical Schools appear to accept a small number of mature students annually as part (5%?) of their intake. Students applying to study medicine require three or four high grade 'A' Level passes, and will usually have studied at least three out of Mathematics, Further Mathematics, Physics, Chemistry or Biology. Some Medical Schools run one year 'Pre-Medical' courses for students who have non-science based 'A'-Levels.