I know he was in a "Welch" regiment, but this could either be Welch Regt or Welch Fusiliers from the way ADRIC register was written. I know he had an MC, but there are these 2 men who satisfy the clues.
George Harold born 8 Nov 1895 in Glamorgan, 2 options for George Henry 1889 or 1895.
The more likely is the man in Welsh Reg rather than Welsh Fus , as register does not give "Fusiliers" only Welch. So I will give his details
1895 Nov 19 Born Glamorgan. George Harold Mills was born on 19 November 1895 in 23 Glanmor Crescent, Swansea, Glamorganshire, Wales. He was the son of Charles Harold Mills and Eugenie Ross.
1911 census at Callincroft Newton Mumbles,
1914 Dec 15. George Harold Mills. Commissioned 2nd Lt in Welch Regt
1914 - 1918 He was gassed twice and wounded three times, according to his grandfather Geroge Mantoch Ross. He was a Lieutenant and received the Military Cross.
1918 Jun 3. MC Gazetted
1920 Oct 28. Joined ADRIC with service no 908. Posted to Depot Coy
1920 Dec 28. "Permitted to resign on medical grounds"
1921 Apr 9. To be temp. Capts.: — George Harold Mills, M.C. in 6th Welch Regt
1952 Apr. George Harold Mills lived at 14 Montpelier Terrace, Hove, Sussex,
1974 Jun Died Surrey