Thousands of Johnstons.....
But could find only one for either 8th or 19th Battalions.
http://www.cwgc.org/search/casualty_details.aspx?casualty=549631Unfortunately it does not give his next of kin details, so at this stage we have no way of knowing if they are related.
I have assumed your grandad wasnt Scottish, or from anywhere other than London area.
The medal cards arent much help, they list only 4 Johnstons for the London Regiment, none of which look likely. it does suggest, though, that if he had a son, whose surname was Johnston, in the London regiment, then that son did not survive, hence my feeling its the lad above.
Also, the badges for the 8th & 19th Bn's are very similar, but the laurel wreath on the 8th badge continues right to the crown, whereas on the 19th badge it stops a little short - your mans badge looks more like 19th Bn to me.
Sorry thats all a bit vague, if we had nok details for F Johnston we would be on more solid ground.
Regards
Pete