Ancestry not only has the Medal Index Card, but also the Regimental Index Rolls.
Duke of Edinburgh's Wiltshire Regiment
25982 Private McCall, Archibald
5th Bn Dorset Regiment, 27987 Private
2nd Wiltshire Regiment, 26982
Transferred to Z class, 2.4.1919 (That's being placed on Reserve)
So, going back to LLT, we find:
Dorsetshire Regiment
5th (Service) Battalion
Formed at Dorchester in August 1914 as part of K1 and moved to Belton Park (Grantham), initially attached as Army Troops to 11th (Northern) Division.
8 January 1915 : transferred to 34th Brigade in same Division.
Sailed from Liverpool on 3 July 1915, going via Mudros to land at Suvla Bay on 6 August 1915.
16 December 1915 : evacuated from gallipoli and went to Egypt via Mudros.
Moved to France, arriving Marseilles 9 July 1916.
Wiltshire Regiment
2nd Battalion
August 1914 : in Gibraltar. Returned to England, landing Southampton 3 September 1914.
September 1914 : moved to Lyndhurst where came under command of 21st Brigade in 7th Division.
7 October 1914 : landed at Zeebrugge.
19 December 1915 : transferred with Brigade to 30th Division.
13 May 1918 : transferred to 58th Brigade in 19th (Western) Division
If we assume that Archibald stayed with the Dorsets through to Gallipolli, he would have joined the Wiltshire's in late 1915 or early 1916?
The 30th Division (
http://www.1914-1918.net/30div.htm) took part in:
1916
The Battle of Albert* including the Division's capture of Montauban and subsequent fighting in Trones Wood
The Battle of the Transloy Ridges*
* the battles marked * are phases of the Battles of the Somme 1916
1917
The pursuit of the German retreat to the Hindenburg Line
The First Battle of the Scarpe**
The Second Battle of the Scarpe**
** the battles marked ** are phases of the Arras Offensive 1917
The Battle of Pilkem Ridge, a phase of the Third Battles of Ypres 1917
1918
The Battle of St Quentin^
The actions at the Somme Crossings^
The Battle of Rosieres^
^ the battles marked ^ are phases of the First Battles of the Somme 1918
The First Battle of Kemmel Ridge^^
The Second Battle of Kemmel Ridge^^
The Battle of the Scherpenberg^^
^^ the battles marked ^^ are phases of the Battles of the Lys
21st Brigade
Brigade joined from 7th Division on 20 December 1915 in exchange for 21st Division
2nd Bn, the Wiltshire Regiment left May 1918