The RSPCA can only advise on how the animal should be cared for, check out the horse on their website which was left with the owner for a further three weeks even though it was skin and bone, infested with worms and lice! -   The owner has to actually sign over the animal to them before they can remove it.
Some might say that their hands are tied, but its the RSPCA themselves who should be campaigning for a change in the law.