Based on HMRC Statistics approximately 1 million employees have a company car, its the 2nd most popular benefit in kind. The most popular benefit in kind is Private Medical Insurance (2.2m employees).
Often employees will change cars or start/stop having fuel for cars during a tax year and the tax on company cars can be significant, you can use this HMRC calculator to assess the the tax.
http://cccfcalculator.hmrc.gov.uk/CCF0.aspx
HMRC have just introduced a new Check and Update Service for Employees so that you can make sure HMRC have the correct information
https://www.gov.uk/update-company-car-details
You may find that it would be better for your employer to give you a loan, beneficial loans up to £10,000 aren’t a taxable benefit.
https://www.gov.uk/expenses-and-benefits-loans-provided-to-employees/whats-exempt
There companies like Maxxia that promote salary sacrifice schemes for cars
http://maxxia.co.uk/asset-finance/car-salary-sacrifice/
steve@bicknells.net