Whilst id highly recommend latin, id need to ask if youre planning on taking chem, physics and the 2 maths to A2, or if youll drop one as an AS. Latin can be very challenging, and if you haven't studied the GCSE, even more so. You need to consider the amount of time you'd have to put in, learning it from scratch. It wouldnt be impossible, however you need to consider whether youre aiming for 4A*'s in your other subjects (in which case i wouldnt do latin, since instead of learning latin, you could be revising your other subjects and securing A*'s). It all depends on how good you are ar managing your own time.
I self-studied Ancient Greek to GCSE and got an A* when taking the exam a year early, so self-studying an additional subject isn't impossible. However GCSE level is completely different from A level. I find Greek to be much easier than Latin, so you'll also have to consider how hard the subject is.
If you decide to go for it, you'd need to be spending a great amount of time studying, since you will be studying for 5 A levels (compare this to the 3 that most people take). Would there be any way of you having a tutor? If so, it'd give you a fighting chance at securing a top grade.
Best of luck! :-D