There is no better time to decide than in Y12 as quite often if you do leave it to Y13 you haven't got enough specific stuff to write on your personal statement (whatever you do, extracurriculars relating to your subject are important – especially if you are considering medicine)
I'd recommend trying to get work experience for medicine if you haven't already to see if it is actually the career for you.
I was considering chemical engineering, medicine, geography and earth sciences in Y11 and I did two days of work experience in a hospital after which I completely ruled out medicine. While I was interested in human biology I think that's simply not enough of a reason to study medicine and become a doctor; from what I saw in the little time I was there it is extremely demanding and you have to be able to handle difficult social situations and you need a ton of endurance – the doctor I was shadowing had only one twenty minute break in one of the days because the hospital was just so busy and doing though I did nothing but walk around I was completely exhausted after all of it. That said, it is a rewarding career at the end of the day but it is important to have had some experience in it to know that you are in the right field.
Also, it is very competitive and from what I have seen with friends who have applied for medicine this year you do need to have some volunteering experience (e.g. in nursing homes) and a wide range of extracurriculars (so your sports stuff is good). Academically your grades are good enough and I don't think you are at a disadvantage without maths.
Again, I'm not actually doing medicine but if you do want to go into medicine the earlier you decide the better, so you can do all the extra reading you need to and other academic extra curriculars like going to lectures or doing online courses etc etc and volunteering for the social aspect of medicine.
For geography, if that is the subject you are predicted an A* in you would have a chance at Cambridge (though that would also be based off the TSA and your personal statement). In terms of careers, whilst it is very direct and secure for medicine (though a lot of study beforehand) geography has a very diverse range of careers. Granted, some may not provide the security of medicine but there are routes into law, finance, business, tourism, politics (Theresa May did geography at Oxford) etc.
If it is physical geography you are interested (like natural hazards and climate change) I'd recommend you look into earth sciences / geology since these focus on the science behind physical geography and planetary science (ultimately, this was the course I applied for!) and that opens up careers in volcanology, space, oil and mining and environmental agencies again.
Good luck with your application next year!