I've found you Major Thornton in 1871. He and his family are living in Dover. His name is Charles Edmund (not Edward). He has two children only identified by their initials CAA daughter born abt 1864 and EWT son. Looking at FreeBMD the daughter is Cornelia Amanda Augusta reg Q3 1863 Derby and the son is Edmund W T reg Q3 1866 Dover. (annoyingly this was after the BMD index changed to give only middle initials). Cornelia is still with the Major in in 1881, Edmund isn't. The wife identified as E A Thornton isn't with them in 1881 either although the Major says he is married not widowed. I can't find a marriage for them, but the wife is born in the East Indies, so maybe married abroad.
So far we have assumed that Mr Stevenson is writing these illustrated letters, but could it be the son? Writing to his father from a boarding school called Oaklands?