London has one of the world's most famous argots. Cockney rhyming slang emerged from the East End during the 19th century, and consists of words and phrases constructed using a rhyme -- a creative process that makes what you're talking about both less likely to be understood by the uninitiated, and more likely to be humorous. To make the dialect still more obscure, the word that was the original object of the rhyme is often omitted. For example, "bread" meaning money derives from a rhyme with "bread and honey" and "ruby" meaning curry derives from "Ruby Murray," a 1950s' singer. Although some words and phrases have entered common parlance -- "barnet," from "Barnet Fair," meaning hair is another -- you're unlikely to hear too much pure rhyming slang as you travel the city.
However, London does have a vocabulary of its own -- some of it derived from or influenced by Cockney, some disparagingly referred to as "mockney," some related to products, places, and produce that are peculiar to the city, and some just plain slang. You may also notice the liberal use of the F-word on London's streets. Although it certainly isn't considered a polite word, its impact on the local listener is more diluted than in most other English-speaking cities.
Below is a glossary of some London words and phrases you may encounter.
bangers -- sausages; usually paired with mashed potato for "bangers and mash"
banging -- good; usually applied to music
barking -- crazy or mad; coined from a former asylum in the eastern suburb of Barking
barney -- an argument or disagreement
bedlam -- madness; as in "the roads are bedlam today"; a corruption of "Bethlehem," an asylum formerly at the corner of Moorgate and London Wall, in the City
black cab -- an official London black taxi, as opposed to a private hire "minicab"; only black cabs are permitted to tout for fares curbside
Boris bikes -- rental cycles that are part of the Barclays Cycle Hire scheme; named after Mayor Boris Johnson, who presided over the scheme's introduction in 2010
butcher's -- a look (from Cockney "butcher's hook"); as in "can I have a butcher's?"
BYO -- short for "bring your own"; a restaurant that doesn't sell alcoholic drinks but will happily open any you bring along, sometimes for a small corkage fee
circus -- a (usually circular) coming together of streets, as at Piccadilly Circus and Finsbury Circus
clink -- a prison; after the former Clink Prison, on the South Bank
damage -- the cost or bill; as in "what's the damage?"
dodgy -- not to be trusted, suspect; as in "that £20 note looks dodgy"
dosh -- money; also "bread" or "dough"
gaff -- home; "back to my gaff" means "back to my place"
G 'n' T -- gin and tonic; often served with "ice and a slice," i.e. an ice cube and a lemon wedge
gastrocaff -- a fashionable cafe that nevertheless serves traditional English fried breakfasts
geezer -- a man; also "bloke" or "fella"
greasy spoon -- the opposite of "gastrocaff": a basic cafe known for fried food
gutted -- extremely disappointed; as in "I'm gutted that Arsenal beat Spurs last night"
IPA -- India Pale Ale; a type of hoppy, light-colored English ale first brewed in the 18th century
lager -- straw-colored, fizzy light beer such as Budweiser and Foster's, served colder than traditional ales (although it's a myth that English beers are served "warm"; they should appear at cool cellar temperature)
liquor -- green parsley sauce served in traditional pie and mash shops
naff -- cheap looking, or unfashionable
Porter -- type of dark, strong ale once popular with London dockers; London brewers Fuller's and Meantime both brew contemporary versions
pint -- both a measure of beer and a general term for having a drink; as in "do you fancy going for a pint later?"
quid -- one pound; "10 quid" or "a tenner" is £10
subway -- a pedestrian underpass; the underground railway is known as "the Tube"
wally -- a type of pickled gherkin, often paired with fish and chips