Dogs Don't Drive!
Your dog may be licenced, but it isn't a licence to drive a vehicle reminds a reader. This person was concerned about the number of human drivers that drive with their pets riding in a position that might interfere. They are correct, it's not a good idea, even with small animals.
A driver must not cause their vehicle to move on a highway if their control over them or their view to the front or sides is obstructed in any manner.
The large dog standing on the rear seat with head and shoulders out the driver's window is certainly a vision obstruction. How could you possibly make a proper shoulder check to the left? Similarly, a smaller dog on the lap could easily move into a position that would entangle it in the steering wheel, probably at the worst possible time.
We wouldn't think of doing this with our children anymore, why do it with a pet? Restraint harnesses are now available to protect your pet in case of a collision if you choose to use them. Keeping your furry friend safely out of your way would be an added bonus.
Oh, the same applies to human passengers. They must not occupy any position in the vehicle that would interfere with the driver's control over any of the vehicle's mechanisms.
- Requirements for Moving Vehicle - Section 195 MVA