Thats correct, well done! The mileage driven would have been a cost had it been in my old car or my new one. So its not a factor in the purchase of a new car only the increase in expenditure over and above.
Or
20k mile limit with 6ppm for any excess
Reclaim for 15k miles for business per year is £5750x4 £23,000
Fuel doesn't really play a part as that would be a cost in a car that was free
Larger engine would lesson the effect assuming poorer economy
But surely you can discard the 15k miles per year for business also, as that £23k would have been also claimed regardless of this car or any other car. Just by using the same logic.