Business mileage for private car if you have company car

We have a salary sacrifice scheme through which we can get EVs, HMRC has clarified that they are to be treated as Business Cars and hence the AER is 9p/mile. For people that rely on charging on public chargers or for long journeys where you charge at least once on a public charger this means people are left effectively out of pocket for every journey. (9p/mile is about 30p/kwh, but chargers are now between 0.70 and 0.85/kwh) The question is: if you also have a private car can you use that and claim for business mileage? Or can you rent a car and claim for the rental cost+fuel? Or would this in both cases create a taxable benefit because you already have a company car that you should use for business trips?

Hi Maccaw,
Yes,you can claim tax relief on your mileage if you decide to use your own private car to make business journeys, rather than those provided through the salary sacrafice scheme.
Expenses and benefits: business travel mileage for employees' own vehicles
Thank you.