QUOTE:
Field Marshall Sir William Robertson has the distinction of being the only Private Soldier to have risen to the very top, becoming CIGS between the wars, he rose to Troop Sergeant Major before Commissioning and included a spell in jail for allowing a prisoner to escape. The only rank he didn't hold was WO1.
Hardly surprising, as WO I did not exist until WO II was created in 1915, by which time he was a general.