Hi Alastair,
A car allowance is just classed as income which the employer pays to you rather than the cost of supplying you with a company car, you are not required to spend the allowance on a car.
The employer may provide you also with mileage allowance payments for your private vehicle, if these are less than the advisory rates, and you are using your own private car for business journeys, then you maybe entitled to claim the difference in advisory milage rates if you meet certain conditions
Expenses and benefits: business travel mileage for employees' own vehicles
Thank you.