I'm not British, so this bit of opinion is pure heresay. Take this as you will, and I concede to an actual Brit disagreeing with me...however, I imagine that a British person, and only a British person, would ever think that Cockney rhyming slang makes a single lick of sense.
Also to be clear, there is sure to be a "Not all rectangles are squares, but all squares are rectangles" situation here. All people who think Cockney rhyming slang makes a lick of sense are British, there is no one outside of the country who would ever call it sensible, making it a sure fire indication of being a Brit, but plenty of Brits don't think it makes a lick of sense. It can prove that you are British, but it can't prove that your not. That fair to say?