If you are booking exams as a private candidate, you will need to look into this approximately six months prior to the exam series. For example, if you are looking to sit your A Level exams in the June series, you would ideally need to have these booked by January so that you can ensure there is a place available for you at your chosen centre.
Also, if you book your exams in advance, this will avoid late deadline fees later on.
Some centres on the list may say that they are not offering exams to private candidates.
Sometimes, it is worth still exploring this centre as an option as the lists on the awarding body websites is not always up to date.
If there are not many centres in your area, sometimes it is worth getting in contact with the British Council. The British Council arranges exams for learners all over the world. These are for numerous awarding bodies.