You need to restrain them with some form of harness from a legal standpoint:
Rule 57
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.
A chest harness on the dog and a seatbelt adapter would be the quickest and safest way. We recently hired a 6m motorhome and clipped our labrador to a floor lashing point as we didn't have any travel seats in the rear. You do then need a non slip surface like a mat as beds or the dog itself will just slide around in the van around the tether point.