You can't claim mileage allowance for travel to your normal place of employment.
So be carefull where your normal place of employment is and that when you designate a place as your normal place of work it wil stand up to scrutiny.
Therefore you will never "claim as a millage allowance for using my car to travel to work " unless it involves travel to a place that is not your normal place of work.
Rather than asking clients to pay you x amount per mile for traveling to them. Better to set a suitable fee for your services and then "compensate yourself" at the appropriate mileage allowance for the out of pocket expenses you had. This is easier to account for and you don't end up with private motor vehicle useage and insurance issues.