If you are a national of an EEA country (EU countries plus Norway, Iceland and Liechtenstein) or of Switzerland, you do not need prior entry clearance to enter the UK.
Visa nationals and non-visa nationals whose programme of study lasts for more than six months must NOT attempt to travel to the UK without a valid Tier 4 student visa.
If you are a visa national and your programme of study lasts for six months or less and you do not intend to take up work (including voluntary work, part-time work, vacation work or a work placement as part of your programme of study) you can apply for prior entry clearance as a 'student visitor' (a short-term study visa). If there is a possibility that you will want to work (or if your programme of study includes a work placement), then you must apply for a Tier 4 student visa.
If you are a non-visa national and intend to leave the UK within six months of starting your studies, and do not wish to seek employment (including a work placement as part of your studies), then you are not required to obtain prior entry clearance. Instead, upon arrival in the UK, you must seek leave to enter as a student visitor by presenting immigration officials with your programme of study (your offer letter). You must also show that you have the financial resources to meet the costs of your stay. However, we strongly advise all students to seek prior entry clearance even if coming for less than six months.