Best advice is to look at the entry requirements for the uni's that you're looking to apply and see what subjects are required.
I have a few friends who applied to courses that require maths but aren't doing Maths i.e engineering and computer science so they've applied to foundation courses (which I assume you don't want to be in this position). It can put you at a disadvantage without Biology as opposed to other applicants with chemistry and biology in their application. Medicine as you said is your dream so why shouldn't you work hard for it?
Your other grades are very good so why is your Biology behind? I've heard Chemistry is a hard A Level yet you're predicted an A/B so why can't you do the same for another science?
If you drop Biology, I personally think you're going to regret it.
A Levels are hard! No one said it was going to be easy xD