Firstly, don't bother doing further maths. Complete waste of your time for the vast majority (if not all) medical schools.
Next, get loads of work experience! In your long summer following your GCSEs, try and get work experience at a school (special needs children esp good), a nursing home, a hospice, a GP surgery (even if you can only get a couple of afternoons) and ideally a hospital. This will be the best opportunity you have to get work experience as once you start your A levels, your spare time will be extremely limited.
I would also say make sure you have 1 long term volunteering commitment (involving caring for others, doesn't need to directly in a hospital/GP). For example for me this is 2 hours a week volunteering at a hospice. Do your best to keep this up through your A levels as this shows commitment to medicine as well as giving you loads to talk about when you hopefully get interviews in year 13.
Finally, keep yourself well. It's very easy to taken into the whole "I must achieve 269 A* GCSEs and 72 A* A levels in every subject know to man" mindset, but the bottom line is, for most medical schools (excluding Birmingham and a few others) you just need an A grade average at GCSE with a handful of A*s, then AAA or A*AA at A level. You literally don't need endless amount of A levels. After this point, it comes down to UKCAT/BMAT and then interview performance.
P.S. I did Chemistry and Biology at A level so you're welcome to message me if you need any advice.