Both law and medicine are competitve. Personally, if I am in your place then I would go for law because it's quite interesting and it gives you a chance to sharpen your mind, strengthen your understanding and gain your experience across humanities and social sciences. It also helps you to be aware of the surrounding around you. You also have the chance to study crime -- why do people commit crime etc.
Medicine is a really good course and you can deffo do it if you are interested in helping with people's lives. It will be a good experience for you however you may have to sacrifice your social life. So I wouldn't pick medicine because I love to socialise and its for 5-6 years which can be stressful. Sometimes you don't have time for things like to spend time with family and friends.
Dentistry is really a good career, it's quite awesome I think. Anyway, I hope this opinion of mine to help you to decide.
Atm your doing GCSES right. So I suggest you to do open-choice for a-levels such as Biology, Sociology etc. I suggest you to focus on your GCSES first then you can decide during a-levels, that's what I am doing. I too love science and I want to work in NHS but I am not sure what career to pick so I will decide during A-levels. Good luck X