The group, all aged in their late teens and early 20s, allegedly flew to Istanbul from Sudan on 12 March in order to assist victims of war in Isis-controlled areas.
The students were born and raised in England, but had been studying medicine in Sudanese capital Khartoum.
Shortly after arriving in Syria, Khider appeared in a propaganda film urging other Britons to travel to Syria and Iraq.
While many Isis films show pictures of masked men posing with guns, the softly spoken son of a doctor appealed for fellow UK doctors to join him in building a new society.
"There is a really good medical service being provided here, lots of hospitals… paediatric hospitals, with specialised doctors," said the former student of Wallington County Grammar School in south London, sitting behind a desk in a wood-panelled office with a stethoscope around his neck.
The son of a doctor, who finished his UMST medical degree in July 2014, added: "Dear brothers and sisters, we as Muslims and as doctors have a great responsibility.
"All you are doing is sitting in the West in the comfort of your homes. Use your skills and come here."