In BT there are 3 options :
Business Needs - you get a allowance (that you never see) towards a car from a very limited choice, so Astra, Focus etc. for lower grade employees.
Company Car - £500 per month allowance that you never see, towards the monthly cost of a car from a list, quite wide choice but no convertibles or performance (e.g. AMG, ///M etc.)...unless you're a senior exec. Generally needs topped up £200 a month to get close to a decent spec.
Cash - literally just a monthly sum, £450 in my case, added to your salary pre-tax, no restrictions on how you spend it. In theory you wouldn't have to spend it on a car at all if you didn't need to.
Different in Vodafone - up to a certain grade you MUST have a company car, you don't gave a choice. But their list is very wide and cheap because of the deals they have with the manufacturers (I guess because they can guarantee x thousand people will have cars) and everyone is cruising round in top spec 5 series, A6s, E class estates etc.
Cash would always be my first option unless doing huge miles.