Take the bird to a vet they are legally obligated to accept it, and any decent vet will accept the bird/animal without question. Wife and I were out cycling one day on coastal route and noticed a young black headed gull being virtually pecked to death by four rather large crows, we phoned RSPCA and they said a vet will take the bird, because they are legally obliged to do so, our local vet was happy to do so.
The only legal obligation a vet has is to take in an injured animal and give it emergency/first aid treatment. If that means euthanasia at that point to stop any suffering then so be it.