My wife's grandfather was in the Territorial Force from 1911 but At outbreak of the Great War, 5 Aug 1914 to 5 Mar 1915 he served at Home, and therefore did not qualify for the TFM although On 1 Sep 1914 he signed papers agreeing that as a territorial he could be sent on overseas service.
There were lots of family rumours of battle in which he served - all wrong as he was only in France from Mar 15 to Jan 16. The only time he might have been gassed (family rumour) was at Loos on 25 Sep 15. Interestingly on 26 Sep 15 he signed papers declining the offer of extending his service. He later joined the RNPS.