The rspca will probably put the dog down if its a stray and injured they wont pay for treatment.
The RSPCA will and do pay for treatment on stray animals. It can often be hard work getting it, but they do. They provide a log number and vet fees (not sure to what extent).
It's not the RSPCA's job to pick up strays so with regards to the original post it shows they're going above and beyond their charity's aims (I won't say "duties" because the RSPCA is a charity not a government office), although in this case the dog is injured, which is probably why they stepped in.
It is the dog warden's responsibility to pick up stray dogs, most only work certain hours as council resources are often limited. After hours stray dogs are in some areas the responsibility of the police, although I've read somewhere this is changing.
As for what will happen to this chap, in most cases stray dogs are taken to council pounds, or boarding kennels with council contracts. They have 7 days to find the owner and then after that in many cases their time is up and they are PTS if they are not taken out by a rescue or reclaimed. In some cases, pounds do their utmost to prevent PTS, but ultimately lots of dogs are put to sleep every day across the country. There are a handful of rescues who do amazing work with their local pounds and transport dogs across the country via a network of volunteers, to suitable rescues where they can be assessed, vaccinated, neutered and then rehomed. This is a huuuuge task, as dogs from the pound are generally unassessed and unvaccinated. They are not ideal dogs to adopt straight out of the pound for these reasons.
If people want to help, I can pass on the links to a couple of boards where this job is done on a daily basis. Transporters and coordinators are frequently needed.
Tomorrow, one of these death row dogs is coming to us as a foster (Seth the Boxer) if we can find transport. We are very lucky in that he has a history so we have some idea of what he will be like in a home. At the moment, transport for this is not arranged and if his time is up tomorrow. He is one of many dogs in similar situations, many aren't as lucky in that they don't have a rescue to back the pound up when their time comes.