Military Chat

If a soldier was serving in India in 1921 would he have filled out a UK census form?

No - completed by (usually) the head of the household to include all persons in the house on 19th June 1921.

He should have been included on Regimental returns.

Plenty of info found by googling ie -

https://www.findmypast.co.uk/help/articles/360009238618-what-information-does-the-1921-census-contain-

https://www.findmypast.co.uk/help/articles/360002676537-how-do-i-search-records-by-category-on-findmypast-

From FMP

The British armed forces are well covered by the 1921 Census. This includes the Army, the Royal Air Force (formed in 1918) and the Royal Navy (including the Royal Marines).

Also -
https://www.greatwarforum.org/topic/293280-1921-census-ew-some-info-on-updates-etc-not-search-queries/page/4/

A special section of the 1921 Census is devoted to the armed forces overseas. There are 35 pieces (volumes) for the Army overseas, 11 pieces for the Royal Navy and one piece for the Royal Air Force. Those for the Army and the Navy are grouped geographically. For instance, there are nine pieces for the Army in Ireland (right across the island of Ireland), and two for the Royal Navy in the Mediterranean. There are pieces covering the British Army in India (RG 15/28116 to 28124 inclusive), the Army of Occupation in Constantinople (RG 15/28110), the British Army of the Rhine in Germany (RG 15/28138) and the peace-keeping British Upper Silesian Force (RG 15/28139). The extensive presence in the Middle-east, especially Mesopotamia (mostly modern Iraq) and Palestine, is also reflected in the census returns.

etc etc

You could try the absent voters list.

FMP has the Britain, Absent Voters Lists 1918-21

Ancestry has the same, but covering 1918-25, 1939