Yeah I mean, you would be pretty hard put to find a school that would accept you for this year I'm afraid. It is certainly doable (I myself homeschooled with just the books and free resources) but does require a lot of dedication especially in one year. Subjects like English especially are harder, because it's more difficult to mark your own exams, though there are things you can do to get them marked anyway. So for those, you'd have to feel a lot more confident with from the start and be able to use exemplar answers well.
Alternatively, there are online schools that you can fast track with in order to learn content in a year. However they range from several hundred pounds, so are very dependent on if you have that sort of money though. Might be worth considering just doing like some subjects with it, and then doing the rest by yourself?
I would say, again if you're motivated, it's definitely doable to get 8s/9s, though I'd definitely say that's more likely in subjects that you can mark yourself (e.g. sciences/math, etc.). It would also require a lot of work to do in one year, but again it's certainly doable (though of course realism depends on you as said).
Also as Muttley said (sorry I would quote but don't want to lose this haha), iGCSEs aren't at all centres, so you'd have to check registered centres accepting private candidates, and email to see if they do iGCSEs. And again as Muttley said they can be much more expensive (sometimes around 4x as much were you to sit it at a state school). If you were to go this route too, or even at state schools (which don't guarantee allowing you to sit it with them either), it's worth emailing a lot of exam centres to see what their prices are (some are quite fraudulent in a way). And of course, depends where you live, you might have to go further abroad.
So thanks for bringing that up Muttley, very important point
Sorry for the length of this incidentally. But hopefully this gives you a better overview of the iGCSE and self-learn route. Best of luck again