Sorry to be argumentative LadyCad (who, me? As if!), but I really must disagree strongly.
In general matters of a dog straying they will refer you to the Dog Warden and take no action. WRT cruelty, they will require proof before acting.
In cases of neglect, responses are ime varied and not always satisfactory. I reported some Border Collies, including a pup, left tied up in a garden in a NE England minus christ knows what winter with just open-fronted wooden kennels with (often wet due to snow) straw bedding for shelter. I had to pull some strings even to get the RSPCA inspector out. She concluded that as there was water and there was evidence that they had recently been fed (empty but apparently recently used food bowls) the owner had no case to answer.
Far worse was the case of 2 GSDs, one quite old, left out with NO shelter. Despite the requests of GSD Rescue UK, those dogs were left to suffer by the RSPCA. The case is appalling - details HERE scroll down for pictures and more info.
In the case Slushy speaks of, "the dog is now showing fear aggression and growls,snaps and barks if you walk past".
The RSPCA would almost undoubtedly kill a dog like that. They would seem him unrehomeable - PLEASE believe me, I work with independent rescue and both know this to have happened many, many times and get involved in rescuing dogs like this one which the RSPCA have openly told owners/concerned individuals that they will take him in, but they will PTS.
I also know rescuers who, with the help of more caring RSPCA workers get dogs "out the back door" of RSPCA rescue and clinics because they are condemned to death. I've even been contacted by an RSPCA inspector and got a dog to the safety of independent rescue because he was about to be PTS for being, in their opinion, "too ugly to rehome".
Those who know me on here are aware that the RSPCA is my bete noire but they know too, I hope, that this is not only with good reason but that I speak from bitter experience and that my reputation as a rescuer is genuine and trustworthy.