Ginger,
Here is the list of all the TISBURY children born 1875-1881 from the 1881 Census Index. If you are sure of his birth date then William is not there.
TISBURY, Eliza Dau <1877> Lond Lond
TISBURY, Arthur William Son <1878> Leic Wilt
TISBURY, Thomas Son <1879> Lond Lond
TISBURY, Alice Dau <1880> Lond Lond
TISBURY, Elbert Elbert Son <1880> Lond Lond
But in case it is the 1878 one, here is the entry:
Dwelling: 3 Market Place
Census Place: Castle Combe, Wiltshire, England
Source: FHL Film 1341489 PRO Ref RG11 Piece 2029 Folio 43 Page 8
Marr Age Sex Birthplace
William J. TISBURY M 43 M Shoreditch, Middlesex, England
Rel: Head
Occ: National Schoolmaster (CM)
Jane Elizabeth TISBURY M 41 F Shoreditch, Middlesex, England
Rel: Wife Handicap: Lunatic
Florence Emily TISBURY 13 F Sudbury, Suffolk, England
Rel: Daur
Occ: Scholar
Blanche Elizabeth TISBURY 11 F Stanford On The Vale, Berkshire, England
Rel: Daur
Occ: Scholar
Louisa Alice TISBURY 8 F Offham, Sussex, England
Rel: Daur
Occ: Scholar
Ethel Clara TISBURY 7 F Liss, Hampshire, England
Rel: Daur
Occ: Scholar
Arthur William TISBURY 3 M Rothley, Leicester, England
Rel: Son
Occ: Scholar
Sarah WILLIAMS U 40 F Chippenham, Wiltshire, England
Rel: Serv
Occ: Nurse
Let me know if you want more details from the Census on any of the other entries.
The Boer War was 1899-1902, so it is certainly possible for him to have been involved.
Regards, Hugh Smith