Reasonably-enough, many of our potential supporters don't see why they should give their hard-earned cash to pay other people's vet bills, and this is often hotly debated on the National Society's facebook page.
We've just had a case that perfectly illustrates why we struggle with the ethics of this:
One of our clinic users contacted our emergency phoneline because her pup had suddenly begun coughing up blood. She was a responsible owner who had paid for vaccinations and did have the money to cover the emergency consultation fee, but she didn't have enough money to pay for tests to establish exactly what was going on; even at our subsidised rates.
Worst case scenario would have been a blood-clotting disorder, which would be hopeless and untreatable. Best case would be some kind of infection, which could be relatively cheap to treat.
BUT without knowing what kind of infection (and whether it actually was something infectious) treatment wouldn't have been possible.
It would unfortunately have been perfectly reasonable for the vet to advise euthanasia on the grounds that there was a 50/50 chance that it was an untreatable condition and any money spent on tests would be wasted.
We agreed that we'd cover the cost of the tests (and then of treatment if the underlying cause was a simple infection), and today the results are back and show that the pup has lungworm. This is potentially fatal if the lungs are badly damaged, but there is a reasonable chance of successful treatment with relatively cheap wormers.
In this instance the owner probably won't be able to pay any of the cost back and will therefore decide to sign the pup over for rehoming. We have to insist on some kind of penalty if the owner can't pay anything because otherwise everyone would say they couldn't afford to pay and we would go bust and close.
But we don't want animals to pay with their lives because their owners can't find money at short notice.
A hundred pounds is an enormous amount of money to someone who is on income support, and frankly it doesn't help all that much to know that the cost would have been more like three hundred pounds at a private vet.
If owners put off basic things like chipping, vaccination and neutering because they're short of cash it may cost us in the long run in terms of strays who can't be returned, animals with preventable diseases and unwanted litters who need to be rehomed.