As far as I'm aware, for most universities as long as you have the core subjects the other GCSEs won't matter much, as long as you studied enough GCSEs, what matters is the subjects you do for your A-Levels. Maybe you could choose subjects you like, or you think might be useful. For studying medicine definitely do the sciences as separate subjects if you can though. I agree with the previous suggestion of doing a language, as it not only looks good on your application or your CV but also might be useful at some point too.
For A-Levels, people typically study 3 or 4 A-Levels, some people doing 5. For subjects, if you can do 3, do Maths, Chemistry and Biology. If you can do four, I think physics is what most people would choose for medicine, and further maths if they do 5. I think most places just want you to have the Maths, Chemistry and Biology though, as long as you have those three for A-Levels you should hopefully be okay to do medicine.
My information might not be the most accurate, this is just what I learned from my friends when they were applying for medicine.
For more specific and reliable information on what you need to do for the university course the best place to find the information is the universities themselves. Each one will want slightly different things, so I'd suggest you look at which universities you want to study at, contact them and explain the situation and ask what they suggest you do and what they want you to have studied.