c-side: thank you for those links - I haven't come across them before, so that's really helpful.
Carmela: thank you very much for your perspective, and all the extra information. I really need to think about all this! I do remember that my aunts said that my grandfather had "run away" from ?an orphanage in Canada because he was treated very badly. Also, I should mention that he may not have been 19 (ie born in 1881) when he joined the army; I suspect he was younger than that (perhaps 17), and also he may not have arrived in England straight from Canada. Interesting to hear your impressions of the photos!
One other thing I will say is: despite all this subterfuge on my gf's behalf, most of the old family rumours about him (eg that he went by different names; that he was an army deserter) have actually proved correct, since I've been researching his past. So I do tend to view these titbits of info (such as the Christian Brothers, and being orphaned) as fairly likely.
Off to ponder! Many thanks, everyone, again.