The guidelines say,
"Car sharing
It is difficult to socially distance during car journeys. You should avoid sharing a car with someone from outside your household or your support bubble unless you can practise social distancing. You can reduce the risk of transmission by:
opening windows for ventilationtravelling side by side or behind other people, rather than facing them, where seating arrangements allowfacing away from each otherconsidering seating arrangements to maximise distance between people in the vehiclecleaning your car between journeys using standard cleaning products - make sure you clean door handles and other areas that people may touchasking the driver and passengers to wear a face covering"