I don't think people are suggesting that you should give up the idea of being an editor - just to point out that it's a bit different in reality from what people often think it's going to be like (there's a clumsy sentence if ever there was one!).
My perspective is a bit different, perhaps. I trained with the SfEP, too. I third the suggestion to do lots of their courses! That's exactly what I did. You might also consider the PTC ones - they're well-regarded and count towards SfEP membership upgrades.
I'm not a writer, nor do I pretend to be one. You obviously need an excellent command of English to be an editor, but I don't agree that you need to be a brilliant creative writer or a published author. (Maybe I could write an amazing novel if I set my mind to it; maybe not; maybe I'll never find out!)
I came at it from a similar place to you except with a far less 'Englishy' background (degree in Fine Art and experience working in admin for non-profits). The only things I had going for me were (a) a good eye for detail, and (b) a love of, and something of a talent for, reading and writing. I hasten to add that although both of those points had been noticed by other people, I'd never done anything about them other than casually thinking in passing, 'Oh, I'm pretty decent at writing' or 'I have a good eye for detail' - I hadn't worked on those skills at all, or attempted to prove myself, or entered any writing competitions, or ... you get the picture. I'd basically checked web copy and dull-as-ditchwater reports for typos.
Fast forward to a few years ago when I was sick of my horrible boss and in a relationship with an entrepreneurial type who thought nothing of setting up his own businesses doing this and that. Would I have the skills to be a proofreader? Hmmm, sounded like something I could turn my hand to. I investigated and found the SfEP.
Taking the SfEP courses really highlighted to me exactly how much I didn't know. Oooooooooh boy, was there a lot I didn't know! It probably took all of a couple of hours for me to realise that 'proofreading' isn't at all what I (and probably 99% of the population) thought it was. Oh. Em. Gee.
Since then I've taken a bunch of their courses and found my niche as a copy-editor. I've worked on both fiction and non-fiction. I've had private clients - individuals and businesses - and worked (as a freelancer) for major publishers. Along the way I've had some lovely feedback from authors and I'm pretty confident in my abilities within certain areas (genres and topics). This was starting from a position of total ignorance, and I have never had any of my own work published.
You need to decide on your specialist areas and pitch in those fields. If you've worked for defence contractors, you've probably got some specific knowledge right there that would be handy for certain types of clients. Knowledge of terminology and industry norms will be a definite advantage. I mean, someone like me, hot as I might be on soft power or Chinese martial arts fiction ;-), would be a poor choice of editor for that kind of work!
It's more difficult to get experience editing fiction, because it's not so likely to be something that ties in with your previous jobs, but if you can pick a couple of genres you like to read and specialise in those, you can target your training and your marketing accordingly. Think about what your USP could be. (For instance, I try to make it clear that I'm happy to edit erotica, pornography, and violence, because many editors don't state that overtly. I'm sure most authors aren't too shy to ask, but it could make the difference that means I get first bite of the cherry.)
You can absolutely plan to become an editor, and you can get into a position where you get enough work to rely on it as your income. Many members of the SfEP do exactly that. I strongly recommend that you join, and take part in the forums there. Sacrifices are required, in a way that they aren't when you just drift into a job, but that's always the way if you decide you want a change of career, especially when going self-employed. You will need to work hard to do your training, to continue your CPD, to promote yourself/your services, and on all manner of things. There is no coasting when you're self-employed, that's for sure. But if you enjoy what you do, it will be worth it!
You obviously do not realise the scope or intricacy of the work - but very few people do until they get started with training or trying to do the work! Don't let that put you off. Find out more, and if you decide it's not for you after all, so be it.
An internship could be a good way to get experience but will probably be very hard to come by. If you take part in some SfEP discussions you will soon notice that they take a dim view of anyone - even the inexperienced, just starting out - undercutting the suggested minimum rates. There is a bit of leeway for newbies, and everyone understands that you might want to do some work free of charge or for a much-reduced rate at first to take your inexperience into account, but this is not expected to be a long-term thing. I think most people there would tell you: get your training first, before you even think about advertising yourself as an editor. Once you have some training under your belt, you will have a fair whack of professional knowledge and should charge accordingly. Essentially ... make sure you can edit to a good standard before you let yourself loose on someone's work. Yes, you'll be inexperienced at first, but the point is that you should be competent to do the job well enough, and as a result you should have confidence in your abilities as well as a willingness to learn and to gain experience.