Buying/leasing cars through the company used to be very tax efficient but changes in the tax system have closed the gap and it is no longer tax effective to do this unless you are buying a company van for business use or are looking to get a car with very low emissions.  
The reason for this is because your company will save some corporation tax if the cost of a car is put through its accounts but then you will be hit with additional personal tax for the benefit of using the company car.  This additional personal taxation for the Benefit in Kind (BIK) usually outweighs the tax saving for the company.  
So, the most tax effective way is to buy/lease the car personally and then claim 45p per mile for business mileage.