I'm still confused. I was trying to find why we say 'Look at the Derby on him' meaning 'Look at the Belly on him'. Well, now I know it is because it means Derby Kelly - rhyming slang for Belly. But why Derby Kelly??
Because, you say, Boiled Beef and Carrotts states 'Put it in your Derby Kell', yes?
But then surely Derby Kell - Derby Kelly - Belly, must already have been around before the song. i.e. the writer put Derby Kell in the song becuase it meant Belly and he didn't just say 'put it in your Derby Kell' and think that'll mean Belly.

So who,or why, did we come up with the Derby Kelly as Belly to go into the song??

Do I make sense?