The Highway Code states:
"When in a vehicle make sure dogs or other animals are suitably restrained so they cannot distract you while you are driving or injure you, or themselves, if you stop quickly. A seat belt harness, pet carrier, dog cage or dog guard are ways of restraining animals in cars." - The Highway Code
When travelling you are responsible for your dog's safety whether you have one dog or many it is your responsibility as an owner to keep them safe and remember to never allow dogs to be a distraction to drivers (your own and other vehicles) or they could cause an accident.