I would think it most appropriate to learn traditional vocal technique, or to "be taught," in a traditional rather than a formalized academic manner. In other words, by spending time with, talking music with, and most importantly singing with, singers whose style you'd wish to emulate, who are more experienced or more deeply immersed in the tradition you admire, in a sort of apprentice-master relationship.

You seem to dismiss the idea that "technique is just acquired by imitation" [emphasis mine], but how else, ultimately, are you going to learn? Ideally, you would hope to be able to engage in some kind of dialogue with the singer(s) you would most like to imitate, and to have them evaluate the quality of your performance.

You are obviously not entirely satisfied by what you've been able to learn at workshops, but could you envision learning more from anyone who has conducted any such workshop that you've attended?

<> If so, seek them out, attend and seriously study their performances, and try to cultivate a relationship where they'll work with you in whatever manner they see fit, no matter how offhand and informal that may be;

<> If not, you may well have exhausted all avenues of possible advice and teaching available to you there Down Under. If none of the traditional singers who perform and conduct workshops at festivals in your area can help you progress any furhter than you have already managed on your own, I wouldn't imagine it likely that the teacher you seek even exists ~ one who combines knowledge of the folk tradition that interests you with the inclination and ability to provide the kind of formalized lessons you've experienced in your classical/jazz/pop voice lessons.

I'd hate to see you travel halfway around the globe only to be disappointed, but the prospects of finding someone in the UK who can provide the combination of skills you're seeking (traditionial authenticity plus educational acumen) might not be much more likely than back home. Of course, assuming that the tradition in which you're interested is British or Celtic, moving to the "homeland" should provide access to a greater number of master vocalists from whom you can continue learning informally, some of whom you can perhaps befriend, etc.