Thomson received his first GB Cap in 2004 as a Junior, representing Great Britain at the European Shooting Championships in the Junior Men's 50M Rifle Prone where he placed 29th. In 2006 he won the Junior Men's 50M Rifle Prone event at the 2006 World Shooting Championships, making him the World Junior Men's Prone Champion. He was also part of the Junior Men's Prone Team who won Britain's first Junior Team Gold in 24 years. Later in the year he went on to become the British Universities Long Range Champion at the 2006 BUSA Smallbore Rifle Championships.