If you want to keep all your options open, it is said that you should take Maths, chemistry and maths or physics for A level. You can still get into medical school without physics/maths but it's slim you can get into the likes of oxbridge and other top universities (But you don't really seem to care about where you will be placed so this should not matter too much).Due to the competitive nature of medicine, an E or U is not going to look good on your application when you are being compared to people with straight A's and perfect grades. I'm not saying you will be straight up rejected because most schools only want 3 A levels (They probably won't consider your 4th A level but seeing a significantly lower grade compared the others is going to raise some eyebrows).
From what I have heard there is no real advantage to taking physics for A level as not much can be linked to biochem. (I'm not a medical student so take this advice as you please). You can make up for a really bad score if you ace the interview and other parts of your application is strong - also by applying to less competitive courses. And yes taking the gap year route definitely works there has been many people who want to get into in medicine who take gap years and are still accepted.
To conclude, if you want to flunk you better be prepared to make up for it in your other A levels and other parts of your application to be considered. And it's better to get A's in subjects you are good at than D's in 'prefered' subjects.