The Leader

In a parallel universe, the alternate version of the Third Doctor (known to humans by the title of The Leader) was the President of the Republic of Great Britain.

Biography
This universe's version of the Second Doctor selected one of the bodies offered to him by the Time Lords after his trial. It is unclear exactly how he siezed the presidency of the British Republic, it is possible that he managed to work his way up through the ranks of the ruling Party by playing factions in the Party against each other (and possibly using hypnosis) until he replaced the previous leader.

His fate remains unknown. Human members of the British government (who were unaware of his alien origins) believed that he died in the volcanic catastrophe that struck Great Britain in the aftermath of I-Day. (PDA: The Face of the Enemy)