Please don't repost the same thread in different forums repeatedly - you only need a single thread in the correct forum (which this one is). The others will be flagged to be closed.
There is no such thing as a "top uni" for medicine - the GMC and the NHS (the latter of which is the only provider of graduate medical training posts in the UK) consider all medical schools equal, and to ensure there is no bias, specialty recruitment in the NHS is blinded to your medical school so they cannot know where you went to medical school. All medical schools also have to meet the same stringent GMC accreditation requirements to ensure they maintain standards.
Many of these questions are answered in detail in the many stickied threads in the medicine forum, noted above. I would strongly suggest reading those first.
Note you only get a single personal statement and it's unlikely you can write a suitable personal statement for both medicine and dentistry. Both courses will also be looking for strong commitment to that specific profession due to the length of the degree and then subsequent training, and so a diluted personal statement referring to both is likely to be a weak one. If you are uncertain between the two professions, look for work experience in both and reflect on your experiences on each, both individually and in comparison to each other, then pick one and commit to it fully.