I've lived in London and the south pretty much all my life, and i'm not sure i've ever heard it used to mean that. It's rhyming slang, right? Not sure how many cabbies are in the market for memory-safe systems programming languages really.
Also in/from the south (though outside London), and was not familiar with that usage at all. A search does turn up some confirmation, though (e.g. https://www.phrases.org.uk/meanings/287275.html).
