Yeah, you can do a short elective in medical school or a longer project once qualified and into specialty training or afterwards.
Can see why you'd want to do this, but remember it had it's downsides
1. Hardly anybody gets to do it
2. The impact you can have is often very marginal due to lack of equipment or funds.
3.Most organisations such as MSF use local medical personnel and UK doctors train them or advise
4. It's often traumatic and grueling work with little reward.
5. Doctors chosen are usually fluent in another language and have volunteering overseas experience
6. Very dangerous,
http://www.msf.org.uk/South_Sudan_violence_20120103.news , in this doctors were attacked and clinics looted
This probs seems a little grim but it's important to be realistic about this kinda stuff.