@"step on" By Greg Andrews
I didn't think that was a "UK'ism" but here comes my definition
"step on" means to dilute for the purpose of obtaining greater sale revenue. The aldulterant can be anything from harmless glucose or water (like "bulking" chicken breast in holland by churning/soaking with water to increase the sale weight of the breast pieces - illegal in the UK, so it's done on the continent), to the harmful scouring powder or battery acid.
Usually applied to hard "Class A" drugs - as in "a bunch of smack heads turned dead in glasgow yesterday when it turned their skag had been stepped on with drain cleaner". Rather than the far more frightening "a bunch of helpless babies turned up dead in China after their milk had been stepped on with an industrial nitrogenous chemical".