Inland Revenue - Mileage expenses are claimable at £0.40 per mile unlimited mileage.
If they, or even you are using private transport, ie not a van, then mileage can be paid at this rate and is tax deductable from your business. When I first started out I had a car and trailer and claimed mileage from myself which was tax deductable, I averaged about £400 - £600 per month in expenses.
I use occasional cleaning staff for builders cleans etc and will pay them this rate to and from jobs, normally only one or two vehicles.