Ah..You mean Reserved Occupation. No there is no list.
In early years of war men were volunteers and conscription only came in in 1916.
http://spartacus-educational.com/FWWconscription.htmYou generally use the term "enlisted" as in signed up. You will also see "Attested" to mean when they fill in the "Attestation Sheet" signing up to serve.
Those pesky Ancestry hints should be taken with a huge pinch of salt. There are loads of Thomas Startup medal cards and no way to match one to your man without more information which we don't have. Sometimes people have a medal with a name and number and regiment on it, or maybe a photo of a man in uniform with identifiable badges, or letters home, or a wartime marriage certificate etc. Without something like that to give you regiment/service number it is impossible to match up a medal card to a man.
Very occasionally a medal card will have an address on it (for where to send medals) so always worth checking the original. (Ideally the Ancestry colour version which has front and back)
There are no quick and easy ways...without knowing a regiment or service number you have to plough through service papers.
Look at Ancestry and if there is no Thomas Richard then you have to open original image for every single possible Thomas Startup looking for age, occupation, birthplace, address, next of kin. Eventually you may find one that matches something you already know.
Only about 30% of papers survived the blitz in WW2 so you still only have a one in three chance of success. That said, I have found people this way.
There is also a thing called National Roll of The Great War. It lists some men who served and often gives their address. The details were supplied by family after the war who paid for the entry. It is a tiny proportion if those who served but I did find my great grandfather there whose records had been lost. I believe you have ancestry
http://search.ancestry.co.uk/search/db.aspx?dbid=9283Milly