I've checked the List of Officers and Men Serving in the First Canadian Contingent compiled probably in early November 1914 and I can see no Rees with No 2 Field Ambulance. There is a John Andrew Rees 34319 with No 2 Stationary Hospital, and L A Rees 28667 is listed in A coy, 16th Battalion.
There are all sorts of anomalies in the roll and on at least a couple of occasions Divisional Orders listed men and asked if it was known if they were with the Contingent!
The Canadians endured terrible weather on Salisbury Plain and on several occasions tents were blown down and paperwork scattered over the countryside. I have several postcards that were forwarded from Canada in the belief that the addressees were with the Contingent and which never reached the intended recipients, and in some cases I can't trace the names. (A batch of these were auctioned locally a few years ago and last year were offered on eBay.)
I found another photo taken in England around 1901 of William. He is much younger in this photo. Is it the same person as in your photo?? What do you think?
I have Williams whole military file and the promotions in Westenhanger on 16/10/14 and 29/1/15 whilst with the No 2 Field Ambulance and Can. Sany Sect respectively. These entries were made though in 1917 to the record!
The transfer to Capt Howlett Larkhill comes from the War diaries January 1915 page 8 of the #2 Field Ambulance. So with what division is Howlett?? Maybe William goes to Larkhill but is not with No 1. I sort of thought he did though when you produced his service number.
This same page of the War diary shows the transfer of William his son. A previous one on Dec 9 page 4 shows when William was "taken on strength"
His Canadian overseas Expeditionary Force Attestation Paper, is stamped right across the top of it with No 2 Field Ambulance. You certainly have stirred up a conundrum with the magnificent photo.
John Andrew (my grandfather) was originally in the No2 Field Ambulance as well but was transferred to the No2 Stationary hospital and was the first to enter France (I have his military file as well.)
Can share more if we can somehow swap email addresses.
do not know the coy L A Rees.
all the best.