things to do to get into a good uni other than getting good gradesWatch
I advise that you don't do NCS. Continue with DofE, take up any roles in school you can, such as an ambassador or anything else. Grades form a reasonable part, but they're not everything. Get your work experience (at least two weeks of highly relevant experience in your chosen trade overall) and aim to build your statement around wider reading and such.
I'd say clubs/activities aren't helpful unless they directly relate to dentistry or you're the leader/captain. Any roles such as Prefect or a similar high ranked position. Work experience is key, maybe show you can work with different age groups, or people with disabilities. And obviously work experience in a dental/orthodontist surgery. Something that shows initiative, for example I had tutoring; you could also do a mini-business.
basically as above. super curricula are important with things like dentistry/medicine/pharmacy/veterinary sciences. try and attend lectures/ broaden knowledge of a chosen field. wider reading is useful. work experience too, but focus on what you learn from it, not how much you do.