Look at the guide which has the table of requirements for med schools. Note how many either have a requirements or direct preference for biology, versus how many are that bothered about a non-science subject. That will answer the biology vs eng lit from a medicine perspective. Given the whole nature of a medical course, you probably want to give biology a try. You're welcome to drop it for A2, as I have a friend who did that and still got in, but english lit isn't going to do you any favours without biology on your side. Their preference would probably be for biology, and english lit, rather than further maths. I'd keep further maths though, as presumably you enjoy it, and it's very much worthwhile for physics.
What is it about medicine? Do you just like the idea, or is it really something for you? I ask this because it's a very "easy" subject to think about doing, it's there on a little pedastool with a career mapped out and everything. Your initial choice of subjects kind of suggests to me physics is probably more a natural route for you, but that's not based on much. Getting into medicine is a massive amount of work, which is enough to put someone off if you're not that fixed on it. I wouldn't want you to feel like you had to stop a subject you love, in order to take biology, to find you actually don't really want to apply for medicine. Have you done any work experience yet? I also think biology will help with UKCAT/BMAT but i'm no expert.
Biology from a physics perspective is again worth more, just in potential for biochemical engineering etc. or any chemistry routes you may go down. It's not really a big deal at all though, so you can go with english lit and not really be closing many doors.