The Vetnurse blog has a long and excellent piece about nursing dogs with parvo-virus. It isn't worth risking a dog's life in order to save the cost of vaccination.
Whether or not animal charities should be prepared to help with the cost of vaccinations for owners on very low income has sometimes been a contentious issue.
On the negative side: vaccinations are a predictable expense, unlike, say, a broken leg. If vaccinations are offered at charity-run clinics and hospitals, they are potentially competing for business with private vets. This is not just unfair to those vets: it might be harming the pet owners who need to use the private vet (by decreasing the number of private veterinary surgeries so that it's more difficult to find a local vet).
On the positive side: £30 is a lot of money for someone on benefits of £60 or £70 a week. It's always likely that they will put off getting their pet vaccinated and just hope they'll get away with it. Providing low-cost vaccinations encourages more people to get their pets vaccinated, and reduces the spread of infections. This benefits other pet owners whose animals may be at risk in spite of being vaccinated, because they're very old, or very young, or have inefficient immune systems.
As a courtesy to local vets RSPCA clinics are expected to consult them before offering a low-cost vaccination service. We did that several years ago when we first started providing vaccinations at the Cambridge clinic and, to their great credit, every one of them backed us in spite of the risk that they might lose money by it.