Phil,
I should just add a rider to my earlier comment on the draft. Men in exempt occupations would be drafted, but they were then allowed to apply for exemption if they wished.
Clearly Frank was not yet a soldier on Christmas Day 1915. If you purchase the birth record of his first child born born q3 1916 that will tell you if by then he was a soldier. Birth records are currently cheap online from GRO (£6)
Unfortunately there were many Frank Holmes who had medal cards. However, if he was wounded he should be on the war pensions list on Ancestry.
Dave

It is getting close to Remembrance Day, the Military records on Ancestry are usually free around November 11th.

Another option is the Great War Forum, you can ask questions about WW1 service records there.

http://1914-1918.invisionzone.com/forums/