The only thing I can suggest is maybe taking a camera in future. I dont know if that might make it difficult tho, if the owner sees you taking pictures then they might not let you take the rabbit :? Of course you could sneak a pic or two when they werent looking
And ask the vets if - out of the goodness of their hearts (ie, free), they would write a factual statement about the rabbit's health/condition when you brought it to them, any treatment given by them and future health care needs if anything.
But clearly the important thing is just to get the poor bunny out, like you did. Well done
I dont see why the RSPCA are being so unhelpful: at the very least they could visit and 'chat' to the bloke and put the wind up him a bit!