Buildersmate has explained the tax rules (which define the maximum you can be paid before having to pay tax). Unsurprisingly those maximum rates (i.e. 40p per mile for the first 10,000 miles per annum and 25p per mile thereafter) are what our MPs receive for their business mileage.
Rates vary widely but JobCentre Plus pay me 20p per mile if I have to attend an interview with them, other than at my local office. When I'm not unemployed, I often work (on a casual basis) for a traffic survey company. They've recently increased their rate from 17p to 19p per mile. (However, the other traffic survey company I work for doesn't pay a penny towards mileage. They do, however, pay a higher hourly rate for the actual job). I also do casual work for the company which has the contract to distribute Yellow Pages throughout the country. They've recently increased their rates, for team leaders, from 20p to 25p per mile.
19p per mile is probably at the lower end of the range offered by employers. However, in most cases, it should be sufficient for what it's meant to do, i.e. pay the driver's additional costs incurred by driving his own vehicle for business purposes. (My car uses petrol at a rate of about 15p per mile. The extra 4p contributes towards oil, tyres, etc). A mileage allowance isn't meant to cover the time element of travelling. This should be paid additionally (and be subject to tax). For example, the traffic survey firm which pays me 19p per mile to get to a job also pays me (at the national minimum wage) for the time taken to get there.
Chris