Adjust the motor expenses in your TB to read "Mileage allowance" and then don't show any other motoring expenses in your TB. Don't forget when you do the SA return, show the addback for private mileage, the Revenue like to see "add-backs". They are less likely to request further information.
If he is completing the HMRC SA return online then it is not possible, they only want the net figures. So show him/her the gross figure, the addbacks and the net figure. It is then up to the client which figure he/she enters.
