Could anyone shed light on these uniforms?

[Neon_strobelight]

Would anyone know what battalions' these uniforms may be from please?

[jim1]

Possibly Army Service Corps.

[MaxD]

Agree.  There is a William Forrest from Edinburgh served in the remounts service of the ASC (hence the horse) whose record is on Ancestry - him perhaps??

MaxD 

[Neon_strobelight]

Thank you but it is a different William Forrest, the photographs are two different people.

[jess5athome]

I tend to agree with Army Service Corps, do you have a name for the chap on the horse?

Frank.

[jim1]

Both are ASC.
There are 2 of that name in this Corps, the one mentioned by MaxD b. 1882 & another b. 1892.He was in Motor Transport. I don't think it's this one as this man looks older & he also has a medal ribbon meaning he was in the Boer War, so still could be MaxD's man.

[Neon_strobelight]

I don't know who the man on the horse is, I was hoping by finding out the regiment, it might narrow it down.

The second photograph I believe is my Great Great Uncle Bill Forrest from the Great War.

[Neon_strobelight]

Thank you, here is the first page of Uncle Bills army record. The armed forces is not my field of expertise, far from it. What exactly was the ASC and what did they do in the Great War?

[MaxD]

He is the man in the Mechanical Transport (MT) element of the Army Service Corps that jim1 cited and indeed is not the Edinburgh remount man I queried.

The link following explains it better than I could but essentially, as he had been an agricultural vehicle driver in civilian life, he was a natural for an MT Company. 
http://www.longlongtrail.co.uk/army/regiments-and-corps/the-army-service-corps-in-the-first-world-war/army-service-corps-mechanical-transport-companies/
As you can see, there were many MT Companies working with different elements of the army.  His record is a bit of a mess but many of the companies are identifiable and can be seen on the link.  To take just one example seen on the next to last page, he is with 594 Company who shifted ammunition for the heavy artillery.

MaxD