If you can make the work experience you did with the dentist relevant to medicine then yes, it helps! You can talk about the similarities and differences and why you wanted to pursue medicine rather than dentistry after the experience - I did this with my pharmacy work experience and it worked for me :')
You're very unlikely to be able to get anything in a hospital at such short notice I'm afraid, but given that you have experience in a GP and on the respiratory ward you should be okay - universities understand how hard it is to get hospital placements. Your voluntary work sounds good, especially if you've been doing it for a while, and that's something that a lot of unis will focus on as it shows "commitment to care" which they seem pretty keen on.
Good luck with UKCAT and the rest of your application!