Bellas girl,
In good military tradition, platoon commanders and platoon sergeants would have been actively ensuring that all their men were accounted for, and ideally kept together. This might also have been happening at the next level up - the company - if possible, but given that at full strength the company consisted of over 100 men, the chances of maintaining a cohesive unit was probably impossible. But as I mentioned it is likely that at platoon level, if not at the level of the individual, as Dyingout's story shows, someone might have been noting which boats or ships the men embarked on, and this detail might later have been recorded in a regimental history.
That applies to sub-units which effectively fought and stayed together, like the infantry, the armoured corps and the artillery. However a large proportion of the BEF was made up of those who provided the necessary engineer, logistic or medical support and were dispersed in smaller groups or detachments. In the main these men would have had to rely on their own initiative to get back to Britain and so the details of how they did this would in most cases be in the memories of those men alone.
Many oral histories of events such as Operation Dynamo were gathered together during and after the war, so it would be worth asking the Imperial War Museum about what they hold by way of recollections.
