British troops served on the Italian front after the Italian defeat in Caporetto. The Italian rout (from the 28^th of October 1917 to mid November) was eventually halted on the Piave River. Meanwhile the concerned Italian allies had resolved, in agreement with Italian Prime Minister V.E. Orlando, to send expedition corps to help secure the situation.

As a precondition the allies demanded the sacking of Italian Chief of Staff Luigi Cadorna; a step Orlando had previously gained the approval of the King for. On November 4^rd a meeting of the newly appointed Government had found an agreement on the replacement of Cadorna with General Armando Diaz. The decision was postponed until after the Rapallo meeting (November 5^th and 6^th ) between Prime Minister Orlando and the French and British representatives, Painlevé and Lloyd George.

This meeting was made more urgent in light of the situation, made worse by the forcing of the Tagliamento line the day before, that had brought the Government front "liason" L. Bissolati to telegraph to the Prime Minister in these terms: situation worsens rapidly with forcing high Tagliamento by enemy. Will attempt resistance on Piave with few hopes. Believe necessary explain full picture to allies. If they believe their interest to resist need send fifteen more divisions on the Mincio [the allies doubted Italy could resist on the Piave; the Mincio River went from the Garda Lake to the Po, through Peschiera and Mantova, the land where the Wars of Italian Independence had been fought]. Otherwise our Country is open to enemy's will and current Government not adequate to discuss surrender. Must be called into government representatives of those who arranged moral dissolution of the army [he means Giolitti].

In light of the allies casting doubt on the Italian ability to resist, Orlando had to reassure them that: the Italian Government had already agreed on the necessary reorganization of the High Command. As it had been established during the last Council of Ministries, which entrusted him and Sonnino with full powers, in agreement with the War Ministry. […] Yet it was necessary to take into account the difficulty of accomplishing such changes in difficult circumstances. The French and British weren't sold on his reassurances.

It was the allies (the British diplomat Maurice Hankey during lunch break) who offered the hesitant Orlando a way to get rid of Cadorna without too much noise; as Cadorna was appointed Italian representative to the Inter-allied Council.

In the following weeks the situation of the front stabilized, while the French and British begun to deploy. It was in fact a contentious issue for a while that the allies did not move immediately to the front line but awaited for the situation to develop one way or another.

Despite Orlando's reassurances to the Chamber, this brought some lasting tension between the Italians ad the allies; the decision not to deploy the French and British division on the front line until after the Italians had secured the Piave positions, caused a widespread resentment among the Italian troops, not helped at all by the observation of the fact that the allies enjoyed better accommodations, rations and equipment. But at least in the first weeks, among the soldiers spread another more troubling voice; that the purpose of the allies expeditions was not to fight with the Italians, but to make sure they wouldn't retreat any further, that in short, the allies were there to point their guns at the Italian's backs. In fairness, this idea wasn't entirely foreign to the French and British commanders either, as General Fayolle remarked that his troops had come “not to save Italy, but to keep it in the alliance”.

Nonetheless the allied corps rose by December to 130,000 French and 110,000 British.

Unfortunately I have no detailed information of their specific order of combat or possible engagements with the enemy, as actions remained mostly defensive on the Italian front until the following Summer, when a large portion of the allied troops had been redeployed on the main front.

Edit: spelling and sources - P. Melograni: Storia politica della Grande Guerra