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Thie index to the Chelsea Pensioner Soldier Service Records, 1760-1920 on Ancestry has this chap - images are on Fold 3 which I can;t access. I note the Harry in Guernsey is in the Yorkshire Regiment which appears to be an infantry set up

Harry Dent
Enlistment Age    18
Birth Date    abt 1871
Birth Place    Bradford, Yorkshire, England
Enlistment Year    1889
Regiment    Yorkshire L. I.
Regimental Number    2909

No. I was talking about her mother, who appears to have the surname Potts in 1901 and 1911 as per Osprey's post, but no marriage could be found

In 1901 the soon to be married Annie has a 10 year old brother John.  Wonder if he's your John?

The military/naval record is likely to be him as it notes place of birth as hackney. DOB is 30 Jul 1884 which just about matches the workhouse and school records found earlier

1901 census he's a boy under training on HMS Lion, a training ship in Devonport

My immediate reaction to Llanelltid was it's a version of Llanilltyd, which tends to be anglicised as Llantwit. The ones I know are  Llantwit Major in the Vale of Glam, and Llantwit Fardre near Pontypridd - a bit too far to be likely here.

However, a bit of rummaging on the web has taught me that the Welsh for Ilston is Llanilltud Gŵyr. Given the link to Parkmill later in life, it looks quite possible?