I can't help with books I'm afraid, and this website largely deals with younger children so is probably not the best place to ask.
In case you are unfamiliar with the situation in UK schools (which may explain why it is hard to find the books you want) please be aware that:
1. There are a number of different examination boards offering most subjects and they will each have their own syllabuses. Are you taking IGCSEs? If so, the number of boards is reduced but there is still some choice.
2. The syllabuses will change from year to year (e.g. language/ literature based exams may have different set texts each year) so be very careful with older textbooks. Details are generally available on the websites of the various examination boards.
3. Many English schools no longer use pupil textbooks at all, or use them in conjunction with other (generally electronic) learning aids. Where pupils do have copies of textbooks they are usually owned by the school not the pupils, so won't be available for resale.Some will, however, have bought their own.
4. Not all courses have an "official" textbook. These seem to be most common for sciences but other subjects will be taught from a variety of sources.
I would suggest that you post a list of the subjects in which you are interested, together with the exam boards you intend to use for each one, on a website aimed at students (not parents, like this one)- for example, thestudentroom.co.uk -and ask recent GCSE students for their advice on the best books to buy. Some of them may also have old copies to sell cheaply. Getting together the materials for 24 subjects won't be cheap though!
There are websites available that help with individual subjects (some require a subscription or password) or with revision/exam technique. I'm not aware of any site that provides full materials for all courses though.
I assume, by the way, that you are choosing to study so many subjects for your own interest and not with a view to attending a British University? Because for University entrance you would be better advised to study a smaller number of subjects at a higher level (AS or A level for example).
Anyway I wish you very good luck!!