Car allowance is taxed, yes (of course)
But a company car is a double hit:
1) you don't get the car allowance (obviously) so that's £350 a month less before tax that you would have had.
2) you also pay 'benefit in kind' tax on a proportion of the new retail price of the car, based on two things, first of all the carbon emissions of the car (HMRC have a league table for cars, which enables you to calculate the figure taxable as 'benefit'), and your marginal tax rate (whatever your highest tax rate is depending on the size of your salary e.g. 23, 40 or 45%).
It's complicated and other than for high mileage drivers or for the self-employed who can fiddle the books, company cars haven't really been tax-effective for years.