Usually 2 word phrase where the second word rhymes with the item in question. Usually (not always) you use the first word to denote the message
eg
2 word phrases:
Brad Pitt = Sh1t ('I'm having a Brad')
Richard the Third = turd ('I'm having a Richard')
Sometimes the full phrase is used, as just the start doesn't scan, eg:
jam jar = car
tea leaf = thief
here's a link to start you off:
http://www.phespirit.info/cockney/slang_to_english.htm
The most common one in London at the moment, I'd imagine is 'it's taters!'
taters = potatoes
potatoes in the mold = cold!!!
vava may confirm if he's hearing this!