I'm proof that GCSEs aren't the end of the world, even for dentistry. I got 3A*s, 4As, 2Bs and 1C (in p.e so I guess it wasn't that important) and I'm a current second year at Birmingham and I've managed to do fairly well.
I had 3 interviews out of 4 as well, the place that turned me down was Bristol and that wasn't for my GCSEs it was for something else. I know entry requirements have changed since I applied back in October 2011 and so I probably wouldn't get in now but basically what I did was I knew my grades certainly weren't perfect so I had to do everything possible in the rest of my application to make sure I got in. I got 9 work experience placements which included specialties, hospital work (maxillofacial and oral surgery), with hygienists, receptionists, lab technicians, NHS and private dental practices (and lots of them!) I went every school holiday I had and started early. Competition for these places is very high. I worked really hard during sixth form and got predictions of A*A*AA (one being in EPQ which I would also recommend doing) and I also did volunteering, DofE etc
Look at the requirements and bring up the rest of your application as much as you can to show you're dedicated and a hard worker and give it a go. You'll always be wondering 'what if?' If you don't!
Feel free to pm me if you want. I'm very into helping people out when it comes to applying etc