From the Mobility site
Car tax, otherwise named Vehicle Excise Duty (VED), road tax or vehicle tax, is something we need to pay to drive and park our cars on roads in the UK. The amount you need to pay out depends on the vehicle and how much carbon dioxide (CO2) it releases.
However, if you have a disability, you may not have to pay VED.
If you’re a Motability Scheme customer, you’ll be exempt from car tax as you’ll already be receiving a qualifying mobility allowance.
The Scheme will arrange for your vehicle to be taxed automatically throughout your lease. However, you can choose to pay tax on your lease car if you want to use your VED exemption on another vehicle.