Please forgive me for reviving this decade old thread, but I would like to share some information that has come my way from the Gypsy Folk, and researches into the 18th 19th Century Cant language, some of which is still in use today.
To 'Buy a Brush' (broom) is to abscond with someone else's property. To this day we talk of a 'Brush' with the law.
Secondly guineas in a 'Slap' refers to a 'Slap Bang Shop'. This is somewhere to buy or fence items. A sort of early money laundering.

Our 'lish young Buy a broom', needs a young man to give her an air of respectability to get her into a lodging house wherein she steals all she can lay her hands on, gets rid in a 'slap house' and offers him a job to do the same abroad. By the way 'Beat my little drum, refers to her rear quarters. No need to explain. Notice the change of Clothes with the ribbons etc. All bought in the 'slap shop' with no questions asked. I think I may have cracked the meaning of this song but my mind is wide open.