Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
With the change of car, this allows you the right to change the way you claim the running cvosts on this vehicle, so you can choose to either stay clamming the 45p a mile (for the first 10,000 miles) or the actual method
This will require you to keep details of ALL mileage (so both business and any private) so a percentage can be established that will be attributable to the claim against all the running costs such as fuel, oil, road tax, insurance, servicing and repairs.
You then also can claim capital allowances against he value of the purchase made - and the percentage you can claim (s unless its an energy efficient vehicle you can not claim the purchase value as such value, but a percentage each tax year)
So I have added a link here for you regarding how the capital allowance is calculated - aqnd what percentages you can claim according to the CO2 emissions of the vehicle so scroll down the rates for cars section
So to recap you claim either
1) The mileage method, 45p a mile for the first 10,000 miles then 25p a mile thereafter OR
2) The actual method, weher you add up ALL the sunning costs of the vehicle and claim the business percentage according to records you keep of business and total mileage - and then also can claim depreciation on the vehicle called capital allowances which are claimed a bit each year according to the CO2 emissions (unless its an enery rfficient vehicle then 100% of the value can be claimed)
Then these two claims on the actual method reduce your tax arising (as they furtehr reduce your net profits) they do not reduce your tax bill directly.
Let me know if I can assist further