Found the Medal Card again (on Ancestry). Image doesn't list a service number, but indicates he was the DSO who was a Major and L/Col with the RFA (Territorial Force).
UK, British Army World War I Medal Rolls Index Cards, 1914-1920Name: Herbert Guy Thomson
Regiment or Corps: Royal Field Artillery
Theatre of War: Mesopotamia
Date of entry therein: March 1915
Note: At the bottom of the 2nd card is a Correspondence address:
Estate Office, Leadenham, Lincoln. I presume it was the address his medals were posted to c1921.
In March 1915, the
5th Hampshire Howitzer Battery RFA left for service in Mesopotamia. The campaign in Mesopotamia, which is now modern day Iraq, was fought primarily between the British Empire and the Ottoman Empire in late 1915, and into 1916.
This link will provide details and photos relating to the 4th Hampshire Regiment in the Mesopotamia Campaign. Keep in mind that Thomson served with the
5th Hampshire Regiment.
https://www.royalhampshireregiment.org/about-the-museum/timeline/mesopotamia-1915/This Wiki link has some good info re the 5th Hampshire Battery in Mesopotamia.
https://en.wikipedia.org/wiki/2nd_Wessex_Brigade,_Royal_Field_Artillery