Rule 57 of 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.”
Depending on your vehicle you could get a dog cage professionally into the boot, this is expensive and take up significant space, however if you were planning on doing lots of long journeys with your pup it is something to consider. I have attached a company who sell crash tested cages Here
Other options are fixing a dog guard, soft material crate or metal dog crate in the boot. these will prevent your dog from distracting you while your driving
The most common type of car safety are seat belt clips, these connect to the dogs harness and will give the dog a little freedom to move around but prevent them from causing a distraction to you. Booster style seats incorporate the seat belt clip but provide the dog there own seat.