Before I went self-employed I ran the editorial department for an academic publisher and one of my responsibilities was recruiting freelance proofreaders, copy-editors and indexers. We got at least 10 applications a week and I'm afraid most of them didn't get onto our list of recommended freelancers, even those with qualifications and experience, simply because we already had so many people on our lists. BUT you've got a technical/scientific background so you have a definite advantage over people who only have experience in "softer" areas like the humanities simply because there are far more people who can handle humanities work than there are who can confidently deal with scientific and technical material. If you want to have a chance of getting taken on by academic publishers I'd say you definitely need a recognised qualification in proofreading. The SFEP are good, as well as The Publishing Training Centre at Book House. I can't vouch for some of the companies who advertise (those adverts that say "Errer" etc) but as an in-house person I never trusted their "qualifications", so it's best to stick with the well-known ones. There's a fund called the Paul Hamlyn Foundation which gives grants to people wanting to train in publishing-related fields, and that would definitely be worth a try if you need help paying for training.
Obviously different companies work in different ways but most of the copy-editing I do (for two main academic clients) is on screen, and documents are exchanged electronically, so there's no printing involved. Proofreading, for the companies I work for anyway, is still done on hard copy (sent out by the typesetter, so again no printing involved on the freelancer's part), though we do send authors pdf files to check. Oh, and ways of working in journals and books usually differ, with journals, especially scientific journals, working, I believe, almost exclusively electronically, with hardly any paper-based work.
Hope this all helps and sorry it's so long. I'm a publishing bore!!