If you happen to be an employer who has workers using their own vehicles for business travel that you subsidize, there are certain National Insurance and reporting obligations to consider.
The Gov.Uk site has a comprehensive five page guide on calculating tax on employee owned vehicles. Rules for working out the tax for this includes understanding the Mileage Allowance Payments (MAPs). Additionally, there are also considerations for deducting and paying National Insurance on employee owned vehicles used for business. The relevant motoring expenditure is calculated based on MAPs as well as other factors including any payments made to the employee towards the vehicle.