I am assuming that you are referring to the private fuel benefit, rather than the car itself. You need to agree with your employer how to pay for your private fuel. This will mean keeping a record of all business mileage and having your employer pay only for that proportion of your fuel. There are a variety of ways this can be done. I pay for all of my fuel, and then charge busienss miles to my employer at a defined rate. Alternatively, if you have a fuel card for example, you could repay personal miles at a defined rate. Or in the past I charged back (or paid for) miles pro rata based on my actual expenditure on fuel. I am fairly sure they cannot force you to pay tax on a benefit you don't want, and which is so blatantly unfair. The level of this tax is such that the tax charged is actually higher than the value of the benefit to many people (me included). I would need to do about 12000 private miles a year to justify the tax (not the benefit, the TAX)
Check this
http://cccfcalculator.inlandrevenue.gov.uk/CCF 0.aspx
out to calculate your tax liability but I estimate it to be about �3,500 pa so if you pay tax at 22% that will cost you about �760 pa or �1,400 if you pay 40%. So at say 10p/mile, that would represent 7,600 or 14,000 private miles.