I think the issue for me is whether the dog got the sensor prescribed by the NHS (they won't have since dogs don't have access to human doctors, medicine or health care organisations)
Also whether getting a sensor for a dog deprives a human of access to that sensor (it won't - unless there aren't enough sensors to go around, which happens rarely, and Freestyle are constantly upping their production to cope with growing demand).
I also see further problems with a sensor or on a dog (furry skin, position of sensor, dog scratching sensor off, etc) but those are for the dog owner to deal with.
From my own point of view, if I had a diabetic dog then I would weigh the pros and cons of cost and convenience against lifestyle factors and inconvenience. Medical costs for a diabetic dog could easily exceed £100/month. But I have no idea if having a sensor would result in a reduction in those costs.
I don't for a second imagine that by putting a sensor on a dog I am impacting a human's access to one via the NHS.