Couple of options here
1) Apply in year 13 with strong A-Level predictions
2) Take gap year and apply with strong A-Level results
3) Study a different degree and go into a graduate medicine course
4) Study something else / Don't go to university
If you're dead set on medicine, then obviously option 4 won't be a great option. However, if you're not passionate about medicine or aren't going to get the grades then it could be the best option.
You can do option 1 and then option 2/3 if you want to. You can apply in year 13 and after a gap year (if you're rejected in year 13).