Of the hundreds of thousands of front line soldiers, only one claims summary executions and even he does not claim to have seen it.
Is every other soldier a liar?
Many memoirs exist. Not one mentions it.
Two that I have read are those of Graves and Sassoon.
Both became anti- war and both are very critical of the establishment.
Graves says that the legal executions were not always reported, but he never mentions summary executions because they did not happen in the British Army.