Atrue Cockney is someone born within the sound of Bow Bells (the bells from St Mary-le-Bow Church in the City of London). However, many born outside this area have a “Cockney” accent, particularly in the East End of London, and in the outer London boroughs, the London suburbs and towns outside London such as Luton, Leighton Buzzard and Romford.
Cockney Rhyming Slang uses expressions that rhyme with the word (instead of using the actual word itself). For example, the word “look” rhymes with “butcher’s hook”. So, if someone says, “Have a butcher’s hook!” it means, “Have a look!” In many cases the rhyming word is omitted. So, this will be shortened to, “Have a butcher’s!” (“Have a look!”).