I knew self-identified Studs and Aggressives when I lived and worked in east and sarf London. Which is not to say there aren't self-identified S/As all over the UK - because I am sure there are. The areas of London I lived, worked and socialized were very culturally diverse and quite queer, and that is probably why it is more familiar to me. As a sweeping generalization, I would say, the majority of people who ID'd as Stud or Aggressive were POC, young-ish (40 and under) & inner-city/urban oriented queers.
But as to your supposition that it is only used by an exclusive minority...hmmmm...I think I would extend that thought and say that I found that gender-related IDs were used by a minority of queers that I knew in the UK, when I lived there. (8years ago now and lots has changed for sure, especially in relation to genderqueer identities.)
I didn't know a lot of queers who claimed Butch/Femme IDs either. There seemed to be an eschewing of "labels" regardless of individuals' gender presentation or desires/personal attractions. I knew a lot of queers who fell soundly to the masculine or feminine side of the gender spectrum but not a whole lot who embraced Butch (as a noun) or Femme (as a noun) IDs.
Perhaps that is why Stud/Aggressive seems a small sub-group to you?
I think there in lies another interesting cultural difference between the US and the UK.
Or perhaps that is where the queer culture in US is moving too, but I'm have a myopic view because of my ID and the places I socialize/find community.