Fuel Mileage Allowance

Is it acceptable for a consultant to claim mileage allowance when travelling to a client’s office? In these circumstances would his home be classed as his business base and then any travelling to clients allowable?   He does not want to fall foul of the rule regarding not claiming travel cost when travelling to work.  Am I correct in presuming that it depends on where his place of work is?    Has anyone come across this issue before and if so I would be grateful for clarification as to whether it is allowable for him to claim that his place of work is in fact his home/office and then any travel is a genuine business expense? He travels from his home to a client’s office some miles away and obviously incurs that cost but is it allowable under tax rules?