This is a difficult question, but a court would probably agree that the financial benefit of having a company car is probably about equal to the car allowance - otherwise everyone who had the choice would choose one rather than the other. Car allowance might be worth less if you do very high mileage as this would cost a lot in depreciation, or worth more if your mileage is very low, but it's really just a question of balancing the net benefit of a company car (no depreciation, loan interest, insurance charges etc but you have to pay tax) against the value (after tax) of the allowance.