The two seated men facing the camera appear to be wearing bandoliers rather than webbing which would mark them out as cavalry, machine gun corps or possibly artillery.
I did a quick google for the order of battle of British XXII Corps which fought in the battle of the Marne and for the 4th & 5th Cavalry divisions which were part of that formation. 1914-1918.net does list the cavalry regiments of those division and has some Indian elements from them on the western front til April, but 2nd Marne didn't kick off til June.
Hope you are able to find out more.