there are two schemes my company has.
1- (the one i'm on). the company leases the car. they pay for insurance, road tax, emergency service cover, all maintance, all car washings. they also pay for private fuel benefit. basically, a percentage of the list price of the car is added to my salary. this is then subject to my rate of tax. the percentage is based on the carbon emissions & type of fuel. i have a toyoto prius, hybrid electric, and my carbon is 104g. one of the lowest. therefore, 11% of the list price is added to my salary. i also get private fuel beneift. again a percentage of the list price is added to my salary. if you go to the inland revenue site, you can find a short pdf that explains this. you can work out what car you want, and the differences in the amount of tax you'll pay.
2- some people want their own car. they get a set monthly car allowance. they also get reimbursed for petrol. they keep track of their work mileage, and they multiply by a set amount to figure out how much they shoudl be reinbursed for fuel.