I'm afraid that if you earn over £43,000, you will pay tax at 40% on your car benefit. All benefits are converted to a cash value for tax purposes. If that benefit wasn't simply tagged onto your income, employers would find all sorts of ways to pay employees in non-cash ways as they did in the 1980s when some employees were paid bonuses in gold or precious stones which they could sell for cash, to avoid national insurance contributions.
It's your choice as to whether you have a company car or not and you need to be aware of the tax implications if your tax rate changes for whatever reason. You could pay pension contributions to bring down your taxable income level or the amount chargeable at 40% to mitigate the problem.
I hope this clarifies your position but let me know if you have any further questions.