while it may be the case that most jobs at fashion houses will be given to fashion graduates, this does not mean that they are any more useful than those without design training. This is merely because nobody has thought to challenge the way designers are educated. If anything, if you have a more hands-on technnical understanding of drafting, cutting and tailoring you are going to be of a lot more use than someone who is just concerned about what they want their personal wardrobe to look like for next season.
One of the shitty things that I have discovered about the fashion industry is that most designers don't have a frickin' clue what they're doing a lot of the time. If they can draw, all they need do is a sketch and send it off to some back room cutter who will first have to convert the sketch into a working drawing (a fashion specification drawing without the frills of a creative sketch - showing silhouette and all features to scale) which will then be taken to the cutting table and developed into a pattern from a block template. In most cases the designer merely has to conceive of the idea and the cutter has to do the leg work. There are many Central Martins graduates who don't even know how to do a working drawing. No joke. So if you can draw at all and you want to do it the easy way, you are already 75% there.
If you really love fashion and you have a deep interest in it, however, you must learn couture and bespoke. This is a craft and a discipline which, once you are good enough at it, will allow you to explore your own ideas in a more technically advanced and rewarding way. You will also gain an understanding of the proportions of the body and fabric.
Fashion education is I am afraid to say totally overrated. So are short courses for that matter and I hasten to add, I know many people who have spent thousands of pounds on DALI courses at the London Institute only to discover that they hadn't learnt anything at all. My advice? Find someone who will teach you how to cut and tailor. If you are already a painter you should be thinking about how you can extend your skills into more technical areas so that once you are good enough, a wonderful synergy between these skills will come of it.
I think there is even someone doing a trial day who posted on the forum. who knows, it might be worth contacting them?