Employee being forced to use own car.

I know an NHS employee who as part of her employment is to be rotated into a community post. The hospital are trying to railroad her into using her own car. It was never in her contract to own a car, They do not pay the business insurance required just the mileage allowance to cover fuel. can the hospital legally force someone to use their own car as the Hospital trust does not have cars available to run the service they wish to provide.

Employee being forced to use own car.