My father was a proof-reader all his life and retired early from a major Uk publisher to take up freelance work. His last job was managing Editorial Services, and I confirm that most publishers have outsourced most of this work to freelancers many years ago.
If you are trying to get into the market, I wouldn't do it this way. I would phone some of the large publishers, ask to speak to Editorial Services and explain what you are wanting to do. That way you will find out how they source their people. A lot of it was done through word of mouth and there was (15 years ago) no shortage of people willing to get involved. My father used to have a 'trial text' to test hopeful applicants - ask. If you have a strong grasp of the English language, you are likely to have gained this from school, and currently feel as frustrated when you read things as Lynn Truss when she wrote the book 'Eats, Leaves and Shoots'. If you understand what I am referring to, then worth proceeding.
I don't believe this sort of language skill can be learned on a short course.