This page explains how you can apply to come to the United Kingdom to take the professional and linguistic assessments board (PLAB) test if you are an overseas qualified doctor.
For details of the requirements you must meet to qualify under this category, see eligibility.
If you are currently outside the United Kingdom, you will need to apply for a visa before you travel to the United Kingdom. Visa applications are processed by visa services, their website provides a visa enquiry form that you can use to decide whether you will need to apply for a visa. If you need a visa, you should make your application at your nearest British diplomatic post.
Visa services, Foreign and Commonwealth Office, King Charles Street, London, SW1A 2AH. If you are outside the United Kingdom you should contact you nearest British diplomatic post.