In mid-October 1914 some 31,000 Canadian soldiers arrived in Plymouth (most of their convoy having been diverted from Southampton because of a U-boat scare) and were taken by special trains to several stations on the edge of Salisbury Plain.
This photograph has been reproduced a dozen or more times, with varying captions. A copy in Libraries and Archives Canada describes it as "Troops of 3rd Brigade proceeding by bus from Plymouth to Salisbury Plain". The play "My Lady's Dress"  opened at the Royalty Theatre on April 23, 1914 and ran for 176 performances, an advert in The Times of Friday, October 23rd announcing  "Last 2 Nights", so it finished on Saturday the 24th, thus greatly limiting the time span of the photo.  The hut or tent in the background, together with the unsurfaced track, suggests that the bus had arrived on the Plain. There appears to be no information about bus journeys in appropriate war diaries.

The bus bears the name,"The National xxxx Car Company Limited", presumably the National Steam Car Company which was established by Thomas Clarkson to run steam buses in competition with the London General Omnibus Co. In 1912 the company was estimated by a competitor to have 27 buses in operation.

The New York Times of October 17 refers to "a long [Canadian] transport train of wagons ... motor trucks and lastly the commandeered London motor 'buses' arriving on the Plain”. Certainly Automobile Machine Gun Brigade No 1 and the Divisional Supply Column did drive to the Plain, via Exeter, Taunton and Heytesbury, an easier route compared with the hilly Exeter-to-Amesbury road.

It is a little difficult to believe that one or more London buses would travel the 200 miles to Plymouth and then 140 miles or more heavily laden to the Plain, and there would have been little need for them to collect soldiers de-training at Wiltshire stations, given the short marchable distances to camps. Unless the bus was already in Plymouth after delivering soldiers from the London area to troop ships there.

 
The obvious answer is that the photo was staged after the event - press photos usually were.

 
That photo certainly looks staged ! no way would passengers be perched or stood in the positions shown on a bus actually moving on rough roads.