For A Level subjects, I recommend doing biology, chemistry and maths because they all 'support' each other in a way and I'm sure you have to do biology and one other science. I'm sure you can't mix certain subjects like geography and biology. I would recommend looking at the entry requirements at several universities and consider emailing them.
About the subjects, I got a 9-8 in science and honestly biology is more of a struggle to begin with (there are a lot of things to memorise and I mean a lot and you need to be able to spell everything correctly) but according to Year 13s, you will learn it automatically with the number of times it comes up. Sample words are erythrocytes (red blood cells), neutrophil (a type of blood cell), polysaccherides.
Chemistry is a lot like GCSE but just goes into slightly more detail and I'm finding it quite easy. The content is structured like GCSE too which makes it much easier.
Now for maths, I got an 8 in GCSE and learning the content is basically the same as GCSE. Content is into more depth but you don't feel the jump compared to other subjects.
I don't know much about dentistry specifically but I am more interested in medicine but I hope this helps