It seems that all the good deals out there are for Diesel cars, very few good deals for petrol C-Class (not talking about C63s...).
The effects of BIK are not straightforward, the overall picture is more involving, especially for the company owner:
With company car, you pay BIK. But you don't pay VAT on the car, and on some of the running costs (servicing etc), and you pay less VAT on fuel. In addition, if the company is profitable, there is also saving on CT which is around 20%-25%, assuming it's a lease that can be posted as OPEX.
With a privately owned vehicle, you pay everything from post-tax income, and you pay VAT on everything, but you can claim up to 45p per me tax-free for business travel.
I actually did the math some 20 years ago and at the time buying privately worked out at the better proposition, though things have obviously changed since.