It depends on the sixth form/college you want to go to entry requirements for a level biology and chemistry. You will not likely be able to do those a levels unless you go on the higher course for combined science. I suggest you asking them if you can do the combined science higher and if that doesn’t work out, tell them how you want to go to dentistry and you want to get in a level biology and chemistry. If they still don’t let you, ask them if you could move up on the higher course if you do well in the foundation so they know you have the potential to get good grades. Btw doing combined science doesn’t limit you from doing a level sciences because I do a level physics and I did combined science: synergy.