Don't do the Chapterhouse one, it's widely regarded as crap within publishing (and outside of it, too).
SfEP is much better. The courses are good. But having done those courses won't necessarily qualify you to work in trade publishing, I'm afraid. The business is awash with freelancers who have years of experience and training, and have all the contacts they need to work as much as they want.
People who self publish books use editors, but tend not to use proofreaders so much. That might be a way into work.