I've always separated her fee - and made sure I have no more than about £15 left in my wallet before I've got out of the car - and always hand it over as she's locking the door behind us, telling her to "Pop that in your purse."
I remember reading on here before about some girls "testing" for forged notes. What would they do if they found a forged £20 & I only have £15 left in the wallet, I wonder? If she refused me service, then I'd give her a 'naming & shaming' on here, for sure.