OK… I gone and done me research now! 
The first Corn Laws were introduced in 1815 in order to ptotect the interests of wealthy landowners in England so to protect the price of home grown wheat after the Napoleonic wars. Irish farmers could not grow wheat because of the peaty soil and ‘The Irish Potato’ famine took place. Robert Peel was Sec of State for Ireland, and he tried to quell the growing unrest in Ireland, by forming a sort of Poilice force but he failed.
In 1822 he became Home Sec and he greatly reformed the penal sytem, but because of growing civil unrest about the price of wheat, he introduced The Metropolitan Police Act. Peel merely expanded on the idea of The Bow Street Runners who had been formed in about 1750, an ununiforned group of runners that went out looking for suspects for Bow Street Magistrates Court.
True Cockneys are said to born in the sound of Bow Bells and this is supposed to be the heart of the ‘Eastend’ The Cockneys were fearful of the new Peelers or Bobbies on their streets. The Eastend was a slum where the common folk lived, and they took umbridge at the fact that the ‘beak’ were so visible, so they started to use codes with each other when they thought that the police were in the vicinity, and from there sprung cockney rhyming slang.
As I have said before, the term ‘copper’ in its widest sense, derives from the old cockney rhyming slang. BOTTLE STOPPER = copper or GRASSHOPPER = copper. The term ‘Grass’ is probably a direct link to the slang of grasshopper (someone who tells tales) as Truckyboy has pointed out, and the fact that they used the term ‘Copper’ in the first place because, as Kate has already pointed out (well researched Kate) that ‘cop’ was an expression meaning to ‘catch’ Therefore they probably took a common word of their time, and rhymed it up!
Phew! Pmsl