For stays longer than 6 months (i.e. full academic year places) you need to apply for a Student Visa.

If you are an EU citizen and are studying for less than 6 months (i.e. for one semester), you can enter the UK via the so-called "Visitors Route" without applying for a visa.

Please be aware that if you plan to take preparatory courses or similar prior to your study visit, you may also need to apply for a student visa, even if you are only studying for one semester.