What I said to someone in your position earlier....
sorry to hear
On the plus side, there is a lot you can do in a gap year...
Firstly, don't let the rejections put your spirits down - competition is really tough so just because you didn't get in does NOT in any way mean you are not good enough. When I got rejections last year it was a really difficult time, so I can completely understand how you feel. Don't worry, you're not the only one. Continue to work hard, and get AAA if not more, then re-apply.
Some of my friends wanting to do med took a gap year, and they all have med offers now.
Do more work experience, go travelling (my friends went to places like China and Fiji), and have fun. Do more voluntary work and improve your application. Practice more for the UKCAT (yes, that stupid test again.. but if you do well, you'll be in a good position).
There's plenty of people who take gap years ... in fact, someone I spoke to from Kings said most of his classmates are gap year students.
Postgrad is harder and more competitive, so in my opinion it's best to take a gap year.
Also, I highly doubt there will be any dentistry places in clearing.
Good luck