The Henry Harry Hubbard born 28/7/1891 was also born Wortham, son of a Harry Hubbard and Alice Collins. His birth not listed as Hubbard as his parents marriage not reg'd until Dec.1891 - he appears to be the Harry Collins birth reg'd Sept.1891 Hartismere. He varies between Henry and Harry in census records but there appears to be no link at all to Mellis via himself, wife, or parents.
I would say that the man you seek is the Harry with Ellen Hubbard in Mellis 1911, shown as visitor. However, having gone back a little bit I cannot see any family relationship between the 2. Ellen was the 2nd wife of a Charles Hubbard - he married (1) Sarah Bennett 1870 Hartismere and (2) Ellen Bennett 1884 Stow. Charles was bc.1848 Mellis whereas Edward, father of Harry (Henry) was born in Wortham and can't see a connection between them.
However, I would definitely think that Harry Hubbard on the Mellis memorial is the one living there in 1911 with Ellen Hubbard (kin or not) who was born as Henry Hubbard (later Harry) in Dec.qtr.1886 Hartismere, mmn Alexander (this being maiden name of Edward Hubbard's wife). Harry could have lived in Mellis for a few years hence being included on the Mellis memorial.
Perhaps his regiment not shown because although they heard he was killed in WW1 he had left Mellis by then. Having gone through the Harry/Henry Hubbards killed in WW1 eliminated all but one (wrong age, wrong birthplace) - Harry Hubbard killed 23/7/1918, Bedfordshire Regiment, no.17679 - all I can find out is that his residence was Hitchin, Hertfordshire and personal effects show he had a widow, Emma. Found a marriage of Harry Hubbard and Emma King 1916 Hitchin BUT the likelihood is that he is the Harry Hubbard birth reg'd 1894 Hitchin so that takes us back to square 1.
I definitely feel he was the Harry with Ellen in Mellis 1911 but what actually became of him I don't know.
Annette