I wouldn't just do as the RSPCA say as it's condemning him to a life as an un-neutered tom for the rest of his life. I think that the priority would be neutering.
Call CP head office on 03000 12 12 12 and tell them you've been in touch with the local branch and get them to contact the branch. As a CP volunteer myself, we do take in injured and sick strays and although they may be full, they need to prioritise an un-neutered cat! They need to help and if it means calling head office I would.
Are there any neighbours who may take on the responsiblity of feeding him?