Jedi,
Daz is right,Using a personal vehicle you can claim the £0.40 for the first 10,000 miles the it drops to £0.25, I've run it this way for the last couple of companies that I've worked for.
If you cover a decent mileage then it can pay off, I averaged 40,000 miles a year so could claim £11,500 back to cover the cost of fuel, insurance and the purchase of the car. I bought a £10,500 car about 4 years ago. I easily covered the purchase and running costs, the car's still in decent nick and I'm quids in.
You need to consider all the impact financially, cash flow etc..