The Visitor visa allows short periods of study for up to 6 months. This visa cannot be extended and work is prohibited (with the exception of students on medical electives).
Depending on your nationality you will either:
- be able to visit the UK for up to 6 months without a visa, however you still need to meet the eligibility requirements and you are still entering as a Visitor when you pass through border control OR
- have to apply for a Visitor visa before you travel to the UK