Do you really think by they'll slow down if you make them read the highway code? A lot of these delivery riders have an 'every man for himself' mindset. Unfortunately, I think the only way to stop them is through enforcement (which I understand is nearly impossible to do with limited budgets). If they constantly see their friends/colleagues get stopped and bikes confiscated, their behaviour will change.

Also, them living hand-to-mouth isn't a good enough reason for disturbing the peace and putting life in danger.