Strictly, an antecedent is defined as the word, phrase, or clause that determines what a pronoun refers to. Applying this definition to your question, it is clear that "these," and other words that are repeated throughout the poem such as "their" and "them," refer not to any of the options you have listed above, but to the soldiers who are dying "as cattle," as the first line describes it. Owen emphasises the ignomonious fate of the soldiers by never mentioning their identity. We are only left with the image in our minds of their mindless and senseless death as they proceed forth and are killed as if they were "cattle." The lack of reference to the soldiers themselves means that we need to piece together what "these" refers to as it is never uttered in the poem.