Complex sentence: Despite being advised by the headmaster at Stamford to consider university, Jackson applied to join the British Army in 1961 and started at the Royal Military Academy Sandhurst in January 1962, graduating on 20 December 1963.
Split into simpler sentences: Despite being advised by the headmaster at Stamford to consider university, Jackson applied to join the British Army in 1961. He was accepted, and started at the Royal Military Academy Sandhurst in January 1962, graduating on 20 December 1963.