These are very interesting observations, thank you. This raises another question as to whether OTB cap badges were those of the Regiment the Officer Cadet came from, or the Regiment to where they were to be posted, or whether the badges were unique to each OTB. The question of no collar badges being worn might be because these men haven't yet been commissioned and posted to a Regiment. I know virtually nothing about how OTBs were run or what their uniform regulations were so thanks for your help so far!
By the way, some of these cadets are wearing 1914 Star ribbons and one has a DCM ribbon so I've been told.