First, make a shortlist of the cars you like from those on offer to you from your company, then work out exactly how much money will disappear from your wage packet in company tax for each car - this is the all-important figure that you have to weigh up against a cash alternative.
Your company tax bill is based on the P11D value of your car. This is the manufacturer's list price (as well as any cost options) including VAT, numberplates and delivery charges. It doesn't include vehicle excise duty (road tax) or the registration fee.
Your firm may allow you to negotiate with its leasing company to get a better car for the budget they've set and the monthly leasing fee your firm has to pay. However, bear in mind that the taxman ignores any such discounts.