Direct proof of use
The L118 Light Gun is directly documented in British and ISAF use during the 2001 War in Afghanistan. MOD image metadata for 28 August 2008 identifies ISAF soldiers from 7th Parachute Regiment, Royal Horse Artillery firing a 105 mm Light Gun at Taliban positions in support of 3rd Battalion, The Parachute Regiment patrols deploying from FOB Zeebrugge near Kajaki, Afghanistan.
A later GOV.UK Ministry of Defence article places a 7th Parachute Regiment Royal Horse Artillery gun detachment in southern Helmand in January 2011. It says the detachment provided offensive fire support for troops on the ground and that Gunner Jonathan Long worked as part of the team that brought into action, maintained, and fired the L118 105 mm howitzer.
Sources: Wikimedia Commons ISAF 105mm Light Gun, GOV.UK Gunners Ready in Helmand
Timeline
The clearest dated firing record is 28 August 2008, when MOD imagery shows 7 Para RHA firing the Light Gun at Taliban positions near Kajaki. The same image was later distributed through Defence Imagery and Wikimedia Commons, with metadata tying the action to Operation Herrick and Helmand Province.
By February 2011, National Army Museum cataloguing places a 7 Para RHA L118 105 mm light-gun crew near Gereshk at Artillery Hill, where British gunners trained and mentored Afghan National Army soldiers. GOV.UK's January 2011 Helmand article documents the same regiment's L118 detachment providing offensive fire support in southern Helmand.
Sources: Wikimedia Commons ISAF 105mm Light Gun, National Army Museum L118 Crew, GOV.UK Gunners Ready in Helmand
Operational role
The supported Afghanistan role is light artillery fire support by British forces within the United States, NATO, and Afghan government side of the war. The documented units are 7th Parachute Regiment Royal Horse Artillery detachments operating with ISAF and British battle groups in Helmand Province.
The sources distinguish combat firing from training and mentoring. The 2008 MOD image metadata records firing at Taliban positions in support of patrols, GOV.UK describes a 2011 gun detachment ready to provide offensive fire support and fire the L118, and the National Army Museum record documents L118 crew activity while mentoring Afghan National Army soldiers near Gereshk.
Sources: Wikimedia Commons ISAF 105mm Light Gun, GOV.UK Gunners Ready in Helmand, National Army Museum L118 Crew