If you're in the UK this association is the sort if thing I'm on about. I have no association with them myself but I've heard good things about them. http://www.cobramartialarts.info/
Here's another thought for you ... Why don't you find a different instructor in something similar, and begin learning from them? If the systems are somewhat similar, you should find yourself advancing quickly because of your prior experience. This gives you the added benefit of continuing to add to your own training, and giving you a deeper insight into the art you're teaching. You benefit, and your students benefit.
Our wing chun lineage hasn't even had rankings set up yet lol. Mostly they focused on teaching and actual skills, but would be useful. I'd say try to get it from a good organization or club after you fulfill their requirement
For what its worth I've been running a pretty successful club now for nearly 20 years without ever telling people or publicizing my grade Honestly it's irrelevant to beginners as long as you have a black belt and people will stay with you as long as you're developing yourself and them
That reminds me of my former Hapkido teacher. After a while I asked him, what Dan he had, and he said "first Dan". He also added, that the grades doesn't mean much, because they mainly show, that you could perform something that day; an opinion all of my teachers so far share and where I grew into as well. Anyway, my point being: A year or so later, we (members of the club) were on a hiking tour and I talked with another instructor; somehow we stumbled over the subject of grades, and he grinned when we were talking about the first teachers grading. A few hours after that, some people were talking about their former Korean master yadda yadda yadda and how my teacher had as one of the few a fourth or fifth Dan. You know, the one that told me, he has a first Dan; I guess technically he didn't lie, as he has a first Dan as well, when he has a higher grade too The second instructor turned around to me in that moment with *that* look and started laughing at me, by that way ^^