Regarding Henry that you have:
Swarbrick Henry, R. Highlanders, Private, 2163
Royal Engineers, Sapper, 137558
To 2 Reserves, 14th January 1919
Theatre of war: France
Date of entry therein: 3rd May 1915There are actually three possible Henrys showing on the 1901 census
...born between 1888 to 1892:
1. Henry, b. 1888, son of Henry and Grace (Pennel) Swarbrick showing as living in Stevenson, Ayrshire in 1901 - born Hamilton
2. Henry, b. 1892, son of William and Mary (Kirk) Swarbrick, born and living in Hamilton and...
3. Henry, b. 1890, showing as son of Jane and born in Tennessee, US (at least 4 of the children born there). Family are living Hamilton!
These boys you would think are all extended family.
Going by entries on family trees again on Ancestry, I think we can take out possibly No.2, Henry son of William and Mary. He looks to have have moved to the US in 1904 (given his age, 12, I would imagine the whole family may have migrated?), dieing there in 1981 (Illinois). There is quite detailed info for him on the US (probably from census data).
Back to the army records for Henry. I don't supposed there was a description for next of kin:
Mrs Jeanie Swarbrick Randolph Street, Cowdenbeath Fifeshire - mother or wife? The fact that in 1901 his mother shows as Jane may help. Maybe a search for deaths for a Jane/jean in Fife would give some indication of age. Don't know at this stage who father was (given he doesn't show in 1901) or Jane's maiden name.
Monica
Added: Mother Mary, Henry and siblings arrived in the US on Jun 26, 1904 on the 'Furnessia' from departure port of Glasgow. They were joining father William who was already there.