My pair used to just hop in the back of the car and curl up on the seat....not sure it was the best way but for the 10minute car journey to the vets that was the only way they wouldn't shout, headbutt and generally cause a disturbance. It was safer for them on the backseat with one of us in the back than throwing themselves around in a crate/back of the car.
But, a crate is fine as long as it's safe and comfy for them (mine were just picky), I think the law is something like they cannot cause distraction or interference to the driver/safety of the vehicle... It also helps if other people can't see, people don't tend to concentrate on driving when they see goats in the back of a car