it varies too much by company to say specifically. Some offer the opportunity to buy your own, others say you have to have a car through their scheme, but with lots of flexibility. Further, others have a VERY strict criteria to follow, even only having one/two choices on their list.
The days of it being a perk are gone really, though its its still cheaper than running your own car, you do invariably have less of a choice.
If you have the car through a companies policy, you generally have an allowance to spend, say £400 a month (which the company pay all of) and then you pay tax based on the purchase price of the car, and its CO2 emissions. The allowance can sometimes be topped up, but you're liable for this for the duration of the car. If you spend less than the £400 allowance you get it back in cash in your salary, which gets taxed. If you can buy your own car with a cash allowance, you generally get more, but get taxed on it, and have to pay servicing and maintenance costs out of that. With a "fleet" car all of this is paid for by the company (tyres, the lot). PCP schemes can incorporate this into the agreement, but then you normally can only do PCP on new cars, and so the budget doesnt go as far.
As an example of how that translates into money, in my job we have to use the companies scheme (HSBC) but can have a VERY wide selection of cars. On the higher tax band you pay 40% company car tax on the emissions. I was going to have a Focus ST, but as the emissions were so high, and with the higher tax band meant the company would be paying the £400 (fine) but my wage would be:
Salary minus:
- income tax
- NI
- car tax
- fuel tax (i choose to have a private fuel paid for too - i pay extra in tax for the "privilige").
For a Focus ST (Petrol, turbo) the fuel and car tax bill each month was ~£430. Which basically means unless you are earning WELL over the threshold you have to have diesel (for any kind of performance). My Alfa 159 TDi for example costs me ~£200 a month