i have a company car here. I also have private fuel benefit. this means that the company pays for all my fuel, not just the fuel i use to do their bidding.
both are taxed as a benefit in kind. so a percentage of the car's value, and a percentage of the fuel i use is added onto my salary. then i pay tax on that amount. (make sense?)
the interesting bit is that the percentage of the car and the percentage of the fuel depends on how clean or dirty it is. For example, my old car was a VW beetle. my percentage was something like 23%. I now have a electric/petrol hybrid Toyoto. that percentage is only 11%.
It's worth it to do a bit of homework b/f you blindly accept a company car in the UK. any more specific questions- just give me a shout (i even have a nifty spreadsheet i made up to figure out my own car tax).