As said above you can use a van then you'll be quids in. It's a base of £3550 taxable value (£3000 for van and £550 for fuel) just times it by your tax rate either 0.20 or 0.40. Compared to your average big company car it's cheaper if you can live with the van drawbacks.
The only thing you have to watch is that it passes the commercial vehicle tax regulations, with regards to windows, no. of seats etc. You basically have to have no rear windows and you're only allowed rear seats if its over a certain weight. Roughly you can have a 5 seat T5 but a 5 seat astra van's a no go. It's a bit of a grey area - you should talk to your accountant, but to be on the safe side it has to be panel van with no rear seats. I've just been through the same thing.
For example a top spec Landrover discovery would cost you £5100 a year @ 20% tax as a company car, but the commercial version with black plastic windows and no back seats would rush you £710 a year, less than the cheapest diesel estate. At the same rate the eco V70 2.0 diesel would cost you £1700 a year in tax. Vehicle sits on the books better for the company as well if bought outright.