If I self teach a subject, where do I take the exam?

Where do I go? And how would I register?

You have to find an exam centre (=any secondary school or college) willing to accept you as a private candidate. There is advice on the exam board websites for private candidates. The cheapest and most likely place to accept you is your current or former school. Entry fees vary wildly between centres so it is worth shopping around. The entry deadline for next year is 21 February with the exam board but some centres that accept private candidates can shut their books by Christmas, so get looking early.