Medicine at a top University will be very hard to achieve, given that most applicants, especially at Oxbridge have 7A*s or more. It's not to say it's impossible, but avoid any extremely picky universities, the top universities are looking for better GCSES for medicine. Even people with 11A*s get turned down. To maximise your chances of doing medicine, do loads of work experience and try to do well in mocks and AS levels if you have them. This shows your improvement which in some cases is just as valuable as sustained success. Going from 11A*s to AAA is far less impressive than going from your GCSE grades to AAA as you have improved so significantly. Dentistry etc should be a lot less competitive and the chances of your GCSE results hindering your chances are quite slim, although this does depend what your high grades are in. Because your 'bad grades' (not bad, just below average for medicine applicants) are due to laziness and carelessness instead of extenuating circumstances, this means you will just have to try extra hard to prove you are a dedicated, hard working student.