London slang

London slang is a mixture of words and phrases originating in the city and around the globe, commonly spoken in London.
As London occupies a dominant social, cultural and economic position within the United Kingdom, slang originally unique to the city has spread across the UK. Conversely, slang from outside London has migrated in along with people seeking work in the capital. Cockney rhyming slang and Multicultural London English are the best known forms of London slang.
Origins.
Slang can infiltrate almost any element of daily life. For instance, London slang about money is believed to have been imported from India by returning servicemen during the nineteenth century. The terms ', meaning £500, and ', meaning £25, are believed by some to have come from old Indian rupee banknotes, which it is asserted used to feature images of those animals, but this is untrue as no Indian banknotes have featured these animals. Confusion possibly arose from the image of an orangutan on the Indonesian 500 rupee banknote. Banknotes with such denominations were issued by Bank of Bengal, Bank of Bombay and Bank of Madras and some other private banks between 1810 and 1860.
It is more likely that the term Pony originates from the Latin "Legem Pone" from Psalm 119, which was always sung at Matins on the 25th of the month. 25 March was also the date on which debts were traditionally settled, on which date one would have to "pony up" or "pony out". It is easy to see how slang for a payment on the 25th could develop into a payment of 25 pounds
The term "monkey" originally meant a mortgage in working class slang across large parts of the UK. A "monkey on the house" or simply a "monkey" was a mortgage.
At that time 500 pounds was a huge sum of money to the poor people who predominantly used such slang and the only way to raise that amount of money would have been to mortgage the house. 500 pounds would have been about the average value of a London house at the time the term originated in the early 20th century according to the CPBS mortgage registers 1919–1922. 
However the true origin of these terms is uncertain. Another money slang word, ', which means £1, is thought to be connected to the American '. "", which describes an unspecified amount of money, may come from the Romany word for coal, "wanga".
Modern influences.
In 2005, Professor Sue Fox from Queen Mary, University of London concluded that Cockney rhyming slang was dying out because children in London are greatly exposed to words and phrases from outside cultures. Teenagers especially are incorporating into their vocabularies new words borrowed from outside the UK. This new slang is also influenced by new technologies, especially mobile phone SMS (short message service) or text messages. While "dat" and "dere" may be of Afro-Caribbean origin along with many other terms, their use in text messages as easier-to-key options to "that" and "there/their" cement them as slang in common usage.
The large number of immigrant communities and high level of cross-ethnic integration mean that various pronunciations, words and phrases have been fused from a variety of sources to create modern London slang. The emerging dialect draws influences from diaspora communities present in London, such as Caribbean speech. This form of slang is mainly spoken in Inner London. Although the slang has been highly influenced by black Caribbean communities, youth of all ethnicities in London have adopted it.