1) Shall I get work experience for both medicine or dentistry?
Both.
2) Which job is more stressful, becoming a doctor or dentist?
Doctor. Less pay, longer hours on the whole, generally more responsibility but depends on specialism.
3) Which pays more?
Dentistry.
4) How competitive is King's College London and Queen Mary's for dentistry?
Competitive, like every other course.
5) In University, which would you find more interesting for learning?
Obviously dependent upon personal interests.
6) What are the benefits/disadvantages of doing dentistry or medicine.
Both pretty rewarding, both good career pathways. Medicine has longer hours, more unsociable. Dentistry is maybe less immediately rewarding than being a doctor in, say, acute medicine? Equally challenging though I'm sure.
7) How long would it take in the UK to become both a dentist and a doctor
5 years PMQ, 2 years foundation doctor/dentist, 4 years doing the other one. A while, and hugely competitive!
8) are dentists undervalued compared to a doctor?
Maybe socially, but not within either profession I wouldn't have thought.