It's the most basic of contracts, you get a lift home and in return the driver gets paid the amount shown on the meter. So why do you think bartering is an option? The time to work out if you've got enough money to pay your fare is somewhere after last orders and before you get into the taxi. Checking your wallet as it turns into your street is not particularly helpful. No, £9.74 will not 'do' for a £15 journey. Neither will £9.74 and a condom. And yes, you will be charged for the trip to back to the cash-point. The one you were stood next to when you prematurely flagged the cab.
