Doug,
Where did you hear about this?
It sounds similar to an 'army travel allowance' claim where you can claim 5p per mile on top of your normal 'motor mileage allowance' when using a private car for an official purpose and take passengers. I think it was 5p for the first passenger and 2p for any subsequent passenger.
Peanuts really.
It's too close for coincidence though, and I don't think you could claim this is you claim 100% of your vehicle and it's running costs from the taxman, only if you claim the mileage.
I'm not sure how it all works, but I'm sure someone will.