Some of the rhyming slang is topical and some lost in the mists of time
Here's some other well used phrases
I haven't gat a Scooby, shortened from I haven't got a Scooby Do, meaning clue.
I'm Hank Marvin. (Shadows guitar player) meaning I'm starving.
You're having a Giraffe mate. meaning you're having a laugh..
The problem understanding is when the phrases get shortened, if you didn't know the original phrase it can be baffling