Why is it that trucking company CR England would charge its drivers a "variable mileage" payment of?

To start...if a person does not like the terms of lease including the mileage expense, then the person is free to choose another company with terms they prefer. There is no one forcing the drivers to stay with this company. Any leased truck here has a mileage payment, so it is not uncommon in my area, though it seems to be new to yours. I imagine all the companies will eventually go to this over time. Change the company you deal with if you are this unhappy.