Can Overseas Doctor can work in UK with Holiday Visa??

I was recently been told by my a doctor friends ( from Asia who currently work in Northern England for couple years ) that doctors here do no require any work permit to work in here! They were under 'visitor visa' when they first entered into this country, and worked in the hospital as a voluntary workers.
I'm doubt about the statements they were given when they told me about this matter.
As I understand than as a visitor, we cannot work in any countries, even we are the professional in certain field! How about the doctor? Is that true that doctor does no require any work permit to work in UK when they first entering into this country and free to work at the hospital???

No. Unless:
http://www.ukvisas.gov.uk/en/howtoapply ... #169326682

No, you can't work as a doctor without the correct work visa.