Ohhk angel, srry it took me too long to actually reply u properly but here's my suggestion:
First option: You can study the other subjects at A2 level and Bio at AS level with ur school, can u not? You may ask ur school about it. And then u can apply for medicine in ur home country explaining to them that u r preparing for ur A2 Bio or u can take a gap year, complete Bio at A2 level and then apply. But u may not waste a whole year doing Bio at A2 level. Whether this option is available to u or not depends on what ur school says regarding this and whether u wud like to opt to do so or not.
Second option: Is there any kind of acceleration programme at ur school to cover A levels in a single year? I hope they won't mind if it's only one A-level subject that u want to study within a year. Can they arrange something of this sort for u? Whether this option is available to u or not depends on whether ur school can arrange such programme for u.
Third option: Like u said, ur school starts within a few days. You can learn a few basics in the meantime, can u not? Get the AS Bio specification, understand wut u need to know, read those texts from the textbook, get the revision guide and have a quick overview. You may learn a lot of things in this way. Familiarise urslf with the content of AS Bio, link the AS Bio areas to the A2 Bio course and read those. Remember my dear that AS is a progression from the IGCSEs/GCSEs so u may find that ur IGCSE/GCSE Bio knowledge and concepts sometimes overlap with that of ur AS Bio knowledge and concepts. It is not that difficult to grasp those basics from the AS Bio course in a matter of a few days although u may not b able to cover the whole specification fully. Later when u start ur A2 with ur school, u can cover some parts of the AS Bio specification regularly. It might appear to b very tough at first and you may need to work extra hard to avoid being left back but it is surely worth it. I will INSHA ALLAH post a few tips on how u may begin to grasp those basics but till then try urslf to learn some concepts.
I hav tried my level best to help u but if u hav any further questions, u may feel free to ask anytime. Hope i helped. Wish u all the best.