Hiya,
My buns were going to be given away free to kids as the breeder didn't like their markings. i took the buns and had a good snoop around at the same time. They weren't even aware of the minimum sized hutch and were feeding them cornflakes. I did report them to the RSPCA. Felt a bit guilty as they were obviously trying their best, but really didn't seem to know anything about them. Anyway, I heard on the grapevine recently that they no longer have any rabbits, so it sounds like the RSPCA did intervene.
Sorry, i'm waffling a bit, but I think it would be best to go through the RSPCA rather than write a letter. The RSPCA are good at educating people as well- it may be that they just need more information.