I keep BWMs and yes you can get 46% KO if they are well grown, but if they are smaller then you may only get low 40s. I don't send my until they are 35kg, ideally more like 38kg. I have mine hung for a week as it intensifies the flavour, but 3 days will be okay.
You won't be able to do much with lambs horns, they will be too thin, and you may not be able to get them back. Skins you should be able to get back but you may be asked if you are registered with AH to receive category 3 waste products (its just a matter of applying on a form and explaining that you want to keep them to be tanned, but you'd need this in advance - ask the abattoir when you book them in if they require this).
Keep your small lambs back and run them on as hoggs. Their taste will be far superior next spring/summer. If I had the space I wouldn't kill any as lambs, I'd do them all as hogget as it's just so lush!