I'm not sure if there is a source for that but this image shows doctors by nationality, not by country that gave them the education to become a doctor. Many of the foreign doctors in that list could easily have emigrated in their younger years or even emigrated for the sole reason of studying in the UK and earning a degree there to become a doctor.

Added to that, it's not the doctor's responsibility to stay in the country they were born in or even the country that gave them the education, specially not if they are going to be poorly paid and forced to work in bad conditions many of those countries probably put them through. It's those government's responsibility to improve the situations of those doctors in their country.

I know it's not easy to do as it is to say, I myself am from Argentina and I personally know a doctor that studied here in a public university and emigrated last year to France because our doctors are poorly paid too and forced to work in horrible conditions, but putting the blame on the doctors is not the answer.