Whilst it is imperative to keep an open mind when it comes to what degree you wish to study in university medicine often differs from other subjects. Although many subjects are very difficult to study in university, medicine requires long-term commitment. Even if you do change your mind about what subject you wish to study in university, work experience and excellent grades will only benefit you.
I often think of the medical application process like spinning plates. One plate is your GCSE results, whilst they are extremely important they are not the only factor to be worrying about. Another plate will be work experience. Another voluntary work, and don't forget about A levels, BMAT, UKCAT and all the interviews! The hard thing is that you can't drop any of the plates if you wish to succeed.
This does seem rather daunting, I know it did for me but the key is to have a good idea of what is required and when you need to do it. For example, what GCSEs are ideal for medicine is key as a first step. The answer to this is really any which you enjoy. Most universities do not discriminate on what subjects you do at GCSE, as long as you get the right grades. This brings me on to talk about how important GCSEs are.
This may be harsh but you need to be realistic. Over 80% of medical students in my year (Bristol) achieved 7A* or more at GCSE. Aside from that, 100% of my colleagues took part in some form of volunteering and/or work experience. It is never to early to start planning!