Some relevant documents - at a price - on this site:
http://www.ww1photos.com/London10th.htmlYour ancestor would have been only 16 at the time of WWI outbreak. He could not be sent overseas until the age of 17 years 6 months, I think. Until then he would have been a boy soldier - tailor or drummer, some such occupation if aregular. But the 10th London Regiment was a part-time soldiery doing some local training only. He may originally have enlisted in the 18th Volunteer Rifle Corps which melded into the 10th later.
All medal awards are on the Medal cards - perhaps your man had a relatively common name.
Another problem is that the whole Brigade went to France in 1916 and remained there until 1919. Your man would have been too young to go in 1916 and may have transferred to another outfit altogether to be guarding at Galata. Actually, as far as I can determine Galata Bridge was in Turkish hands throughout and Galata was the site of a prison which held British and allied prisoners of war. I think the Turks only surrendered that in about October 1918.
Best idea is to put his full names here and perhaps someone can then give you real help.