We get a fairly meagre £400/mth allowance, but have almost complete flexibility on how we spend that.
I could take that, net at £240/mth and buy my own. Or, with our latest HMRC approved scheme, add my allowance to my salary and then pay for the lease through salary sacrifice pre-tax.
Currently my E220 estate costs me £620/mth - £330 of that is BIK as it's rated at 194g/km for emissions, rest is what I add in above the allowance (and is offset against the BiK calculation).
My new quote for an E220 CDi Saloon (with a few toys) is £695/mth - off that comes the allowance so the cost to me (tax, contribution, etc) is now under £300/mth. Most of that comes from the emissions coming down to 132g/km, but I'm also not paying 40% on my contribution either.
Considering that this is for 2 years, tax, insurance, maintenance, I don't think I could do any better doing my own thing.
If I wasn't in the car scheme, I could also choose to lease a vehicle through the same route but obviously wouldn't benefit from the car allowance.
I will probably average 18k/yr, as I now work from home, with about 7k business miles a year.
If your company doesn't operate a salary sacrifice scheme for company cars, it might be worth asking them to investigate it as it seems to work very well for us - we also use 2 lease companies for a bit of competition as well as having about 4k cars in the fleet, so our buying power is quite considerable too.