I took an extra GCSE in year eleven. I simply taught myself the syllabus, went to the exams officer and asked him to enter me for it. He entered me, but said I might need to pay about £30. In the end, they didn't get me to pay for it because they wanted to be able to say that someone had done that GCSE, but I think that they would for History simply because lots of people do it. The bonus of that is that you can easily find cheap textbooks.
Ask your exams officer or History teachers at school. It might be an idea to try and learn a bit of it now, and then ask them in October or November, at the end of the first half term, so that you can show you've already put work into it. Otherwise they might think you're just going to sit around doing nothing. That way, you can also get them to let you do a mock at the same time as the students who have lessons in it. If you get a decent grade, on a par with the other students, they'll probably be ok with entering you for the exam.
Normally exam entrance fees are around £30, I think, but you'll have to get textbooks too, or you could use BBC Bitesize. I personally didn't bother getting a textbook for Latin and just used online stuff, but I haven't got my results yet so I'm not sure how that will end up.