Graduate entry for medicine is even more competitive than the 5 year course. A nursing degree will provide you with tonnes of great experience that many other applicants will not have, however I'm not convinced that it will provide you with the firm academic platform that they are looking for in medicine applicants. Don't forget that most universities will still want the a-levels required for medicine. Maybe you should look at access to medicine courses, or even foundation courses for medicine? Personally, I think that you would be far better to finish your a-levels, do an academic degree (doesn't necessarily have to be science - a good degree in subjects like psychology are also valuable) and then to apply for graduate entry medicine if that's the kind of route you're looking to take.
Now, this next part I say very tentatively. Many people underachieve in a-levels/gcses etc due to personal circumstances, or because they just didn't work hard enough. However If you have not achieved the GCSEs or a-levels because you don't have the level of academic ability required then I would strongly advise that you try to find something else that you might enjoy instead of medicine. For example, does medicine appeal to you because you like the idea of job where you can care for and help people? If so, then maybe you would truly love a career such as nursing. At the end of the day, all entry routes to medicine will be competitive. There is no easy way in, there is no loophole. I would hate for you to spend all your time and efforts doing courses and degrees for the sole purpose of getting into medicine only to fail.
However, in spite of all of what I've just said - if you truly believe that you have what it takes to do this, then you should chase your dream and don't let it go. You can have anything if you want it enough.