I think we are both coming from the 'use our own cars and company pay for the fuel' side of the fence, where the amount of business miles has a very real effect on the tax bill!!
There are generally two options
use your own car, and put in expense claims/company pays for business related mileage at whatever rate it wants (you can then claim the difference if any back from the IR up to the figures above.. 10k etc etc)

or my option..use your own car and company pay all fuel with fuel card, submit that amount the IR upon when I call them and say 'hey chaps, actually xxxx miles were business from that lot' and the reimburse me at the same rates as above.

So, keeping my business mileage up is a good thing in my case - and not hard since I am here and clients are in London. 500 miles every trip ! (down side is car has done 40k in 10 months