I think it would depend on what the RSPCA is like in your area. I don't have any experience with them as I'm not in England, but I've heard some branches put down rabbits with pasturella. Is there another rabbit rescue locally that you could approach? They might not be able to help directly but they may know more about the local RSPCA's policies, and what is likely to become of the rabbits.
Is there any way you could take on the rabbits yourself? Or work there for a short time each week on a voluntary basis? It sounds like a difficult situation.